diff --git a/dist/primality.js b/dist/primality.js
index b3fe171..c3fef3d 100644
--- a/dist/primality.js
+++ b/dist/primality.js
@@ -1,5 +1,5 @@
 /*!
- * primality v1.5.5
+ * primality v1.5.6
  * (c) 2012–2013 Kenan Yildirim
  *
  * Includes functions from Lo-Dash
@@ -93,6 +93,24 @@
     var primality;
     var WILSON_PRIMES = [ 5, 13, 563 ];
     var _ = require("./lib/util/");
+    function factorial(value) {
+      return value === 0 ? 1 : value * factorial(value - 1);
+    }
+    function mod(x, y) {
+      if (y > 0) {
+        if (x > 0) {
+          return x % y;
+        } else if (x == 0) {
+          return 0;
+        } else {
+          return x - y * Math.floor(x / y);
+        }
+      } else if (y == 0) {
+        return x;
+      } else {
+        throw new Error("Cannot calculate mod for a negative divisor");
+      }
+    }
     function leastFactor(n) {
       if (n === 0) return 0; else if (n % 1 || n * n < 2) return 1; else if (n % 2 === 0) return 2; else if (n % 3 === 0) return 3; else if (n % 5 === 0) return 5;
       var m = Math.sqrt(n);
@@ -138,8 +156,8 @@
       if (!primality([ a, b ])) return false;
       return true;
     }
-    function isWilsonPrime(a) {
-      return _.contains(WILSON_PRIMES, a);
+    function isWilsonPrime(value) {
+      return _.contains(WILSON_PRIMES, value) ? true : 0 === (mod(factorial(value - 1) + 1, value) === 0);
     }
     primality.VERSION = "1.4.0";
     primality.areTwinPrimes = areTwinPrimes;
diff --git a/dist/primality.min.js b/dist/primality.min.js
index 3255967..1b3aba3 100644
--- a/dist/primality.min.js
+++ b/dist/primality.min.js
@@ -1,5 +1,5 @@
 /*!
- * primality v1.5.5
+ * primality v1.5.6
  * (c) 2012–2013 Kenan Yildirim
  *
  * Includes functions from Lo-Dash
@@ -9,9 +9,10 @@
  */
 !function(a){function b(a,c,d){var e=b.resolve(a);if(null==e){d=d||a,c=c||"root";var f=Error('Failed to require "'+d+'" from "'+c+'"');throw f.path=d,f.parent=c,f.require=!0,f}var g=b.modules[e];return g.exports||(g.exports={},g.client=g.component=!0,g.call(this,g.exports,b.relative(e),g)),g.exports}b.modules={},b.aliases={},b.resolve=function(a){"/"===a.charAt(0)&&(a=a.slice(1));for(var c=[a,a+".js",a+".json",a+"/index.js",a+"/index.json"],d=0;d<c.length;d++){var a=c[d];if(b.modules.hasOwnProperty(a))return a;
 if(b.aliases.hasOwnProperty(a))return b.aliases[a]}},b.normalize=function(a,b){var c=[];if("."!=b.charAt(0))return b;a=a.split("/"),b=b.split("/");for(var d=0;d<b.length;++d)".."==b[d]?a.pop():"."!=b[d]&&""!=b[d]&&c.push(b[d]);return a.concat(c).join("/")},b.register=function(a,c){b.modules[a]=c},b.alias=function(a,c){if(!b.modules.hasOwnProperty(a))throw Error('Failed to alias "'+a+'", it does not exist');b.aliases[c]=a},b.relative=function(a){function c(a,b){for(var c=a.length;c--;)if(a[c]===b)return c;
-return-1}function d(c){var e=d.resolve(c);return b(e,a,c)}var e=b.normalize(a,"..");return d.resolve=function(d){var f=d.charAt(0);if("/"==f)return d.slice(1);if("."==f)return b.normalize(e,d);var g=a.split("/"),h=c(g,"deps")+1;return h||(h=0),d=g.slice(0,h+1).join("/")+"/deps/"+d},d.exists=function(a){return b.modules.hasOwnProperty(d.resolve(a))},d},b.register("primality/primality.js",function(a,b,c){function d(a){if(0===a)return 0;if(a%1||a*a<2)return 1;if(0===a%2)return 2;if(0===a%3)return 3;
-if(0===a%5)return 5;for(var b=Math.sqrt(a),c=7;c<=b;c+=30){if(0===a%c)return c;if(0===a%(c+4))return c+4;if(0===a%(c+6))return c+6;if(0===a%(c+10))return c+10;if(0===a%(c+12))return c+12;if(0===a%(c+16))return c+16;if(0===a%(c+22))return c+22;if(0===a%(c+24))return c+24}return a}function e(a){return l.isNaN(a)||!l.isFinite(a)||a%1||a<2?!1:a!==d(a)?!1:!0}function f(a,b){return 2!=Math.abs(a-b)?!1:j([a,b])?!0:!1}function g(a,b){return 4!=Math.abs(a-b)?!1:j([a,b])?!0:!1}function h(a,b){return 6!=Math.abs(a-b)?!1:j([a,b])?!0:!1
-}function i(a){return l.contains(k,a)}var j,k=[5,13,563],l=b("./lib/util/");j=function(a){if(null===a||""===a)return null;if(l.isArray(a)){for(var b=0,c=a.length;b<c;b++)if(!e(a[b]))return!1;return!0}return e(a)},j.VERSION="1.4.0",j.areTwinPrimes=f,j.areCousinPrimes=g,j.areSexyPrimes=h,j.isWilsonPrime=i,(c.exports=j).primality=j}),b.register("primality/lib/util/index.js",function(a,b,c){var d={};d.contains=b("./contains"),d.isArray=b("./isArray"),d.isFinite=b("./isFinite"),d.isNaN=b("./isNaN"),c.exports=d
-}),b.register("primality/lib/util/contains.js",function(a,b,c){function d(a,b){for(var c=-1,d=a.length;++c<d;)if(a[c]===b)return c;return-1}function e(a,b){return d(a,b)>-1}Math.max,c.exports=e}),b.register("primality/lib/util/isArray.js",function(a,b,c){var d=RegExp("^"+(Object.prototype.valueOf+"").replace(/[.*+?^${}()|[\]\\]/g,"\\$&").replace(/valueOf|for [^\]]+/g,".+?")+"$"),e=d.test(e=Array.isArray)&&e,f=e||function(a){return a?"object"==typeof a&&toString.call(a)==arrayClass:!1};c.exports=f
-}),b.register("primality/lib/util/isFinite.js",function(b,c,d){function e(a){return h(a)&&!i(parseFloat(a))}var f={"boolean":!1,"function":!0,object:!0,number:!1,string:!1,undefined:!1},g=f[typeof global]&&global;!g||g.global!==g&&g.window!==g||(a=g);var h=a.isFinite,i=a.isNaN;d.exports=e}),b.register("primality/lib/util/isNaN.js",function(a,b,c){function d(a){return"number"==typeof a||f.call(a)==g}function e(a){return d(a)&&a!=+a}var f=Object.prototype.toString,g="[object Number]";c.exports=e}),b.alias("primality/primality.js","primality/index.js");
-var c={"boolean":!1,"function":!0,object:!0,number:!1,string:!1,undefined:!1},d=c[typeof exports]&&exports,e=c[typeof module]&&module&&module.exports==d&&module,f=c[typeof global]&&global;!f||f.global!==f&&f.window!==f||(a=f),"function"==typeof define&&"object"==typeof define.amd&&define.amd?(a.primality=b("primality"),define(function(){return b("primality")})):d&&!d.nodeType?e?(e.exports=b("primality")).primality=b("primality"):d.primality=b("primality"):a.primality=b("primality")}(this);
\ No newline at end of file
+return-1}function d(c){var e=d.resolve(c);return b(e,a,c)}var e=b.normalize(a,"..");return d.resolve=function(d){var f=d.charAt(0);if("/"==f)return d.slice(1);if("."==f)return b.normalize(e,d);var g=a.split("/"),h=c(g,"deps")+1;return h||(h=0),d=g.slice(0,h+1).join("/")+"/deps/"+d},d.exists=function(a){return b.modules.hasOwnProperty(d.resolve(a))},d},b.register("primality/primality.js",function(a,b,c){function d(a){return 0===a?1:a*d(a-1)}function e(a,b){if(b>0)return a>0?a%b:0==a?0:a-b*Math.floor(a/b);
+if(0==b)return a;throw Error("Cannot calculate mod for a negative divisor")}function f(a){if(0===a)return 0;if(a%1||a*a<2)return 1;if(0===a%2)return 2;if(0===a%3)return 3;if(0===a%5)return 5;for(var b=Math.sqrt(a),c=7;c<=b;c+=30){if(0===a%c)return c;if(0===a%(c+4))return c+4;if(0===a%(c+6))return c+6;if(0===a%(c+10))return c+10;if(0===a%(c+12))return c+12;if(0===a%(c+16))return c+16;if(0===a%(c+22))return c+22;if(0===a%(c+24))return c+24}return a}function g(a){return n.isNaN(a)||!n.isFinite(a)||a%1||a<2?!1:a!==f(a)?!1:!0
+}function h(a,b){return 2!=Math.abs(a-b)?!1:l([a,b])?!0:!1}function i(a,b){return 4!=Math.abs(a-b)?!1:l([a,b])?!0:!1}function j(a,b){return 6!=Math.abs(a-b)?!1:l([a,b])?!0:!1}function k(a){return n.contains(m,a)?!0:0===(0===e(d(a-1)+1,a))}var l,m=[5,13,563],n=b("./lib/util/");l=function(a){if(null===a||""===a)return null;if(n.isArray(a)){for(var b=0,c=a.length;b<c;b++)if(!g(a[b]))return!1;return!0}return g(a)},l.VERSION="1.4.0",l.areTwinPrimes=h,l.areCousinPrimes=i,l.areSexyPrimes=j,l.isWilsonPrime=k,(c.exports=l).primality=l
+}),b.register("primality/lib/util/index.js",function(a,b,c){var d={};d.contains=b("./contains"),d.isArray=b("./isArray"),d.isFinite=b("./isFinite"),d.isNaN=b("./isNaN"),c.exports=d}),b.register("primality/lib/util/contains.js",function(a,b,c){function d(a,b){for(var c=-1,d=a.length;++c<d;)if(a[c]===b)return c;return-1}function e(a,b){return d(a,b)>-1}Math.max,c.exports=e}),b.register("primality/lib/util/isArray.js",function(a,b,c){var d=RegExp("^"+(Object.prototype.valueOf+"").replace(/[.*+?^${}()|[\]\\]/g,"\\$&").replace(/valueOf|for [^\]]+/g,".+?")+"$"),e=d.test(e=Array.isArray)&&e,f=e||function(a){return a?"object"==typeof a&&toString.call(a)==arrayClass:!1
+};c.exports=f}),b.register("primality/lib/util/isFinite.js",function(b,c,d){function e(a){return h(a)&&!i(parseFloat(a))}var f={"boolean":!1,"function":!0,object:!0,number:!1,string:!1,undefined:!1},g=f[typeof global]&&global;!g||g.global!==g&&g.window!==g||(a=g);var h=a.isFinite,i=a.isNaN;d.exports=e}),b.register("primality/lib/util/isNaN.js",function(a,b,c){function d(a){return"number"==typeof a||f.call(a)==g}function e(a){return d(a)&&a!=+a}var f=Object.prototype.toString,g="[object Number]";c.exports=e
+}),b.alias("primality/primality.js","primality/index.js");var c={"boolean":!1,"function":!0,object:!0,number:!1,string:!1,undefined:!1},d=c[typeof exports]&&exports,e=c[typeof module]&&module&&module.exports==d&&module,f=c[typeof global]&&global;!f||f.global!==f&&f.window!==f||(a=f),"function"==typeof define&&"object"==typeof define.amd&&define.amd?(a.primality=b("primality"),define(function(){return b("primality")})):d&&!d.nodeType?e?(e.exports=b("primality")).primality=b("primality"):d.primality=b("primality"):a.primality=b("primality")
+}(this);
\ No newline at end of file
diff --git a/primality.js b/primality.js
index 6b74bc6..5255c84 100644
--- a/primality.js
+++ b/primality.js
@@ -13,6 +13,45 @@ var WILSON_PRIMES = [5, 13, 563];
 
 var _ = require('./lib/util/');
 
+/**
+ * Returns the factorial of `value`
+ *
+ * @private
+ * @param {Number} value
+ * @returns {Number} The factorial of `value`.
+ */
+function factorial(value) {
+  return value === 0 ? 1 : value * factorial(value - 1);
+}
+
+/**
+ * Returns the modulus of two numbers.
+ *
+ * @param {Number} x
+ * @param {Number} y
+ * @returns {number} res
+ * @private
+ */
+function mod(x, y) {
+  if (y > 0) {
+    if (x > 0) {
+      return x % y;
+    }
+    else if (x == 0) {
+      return 0;
+    }
+    else {
+      return x - y * Math.floor(x / y);
+    }
+  }
+  else if (y == 0) {
+    return x;
+  }
+  else {
+    throw new Error('Cannot calculate mod for a negative divisor');
+  }
+}
+
 /**
  * Finds the smallest factor of `n`
  *
@@ -153,21 +192,23 @@ function areSexyPrimes(a, b) {
 }
 
 /**
- * Checks if `a` is a Wilson prime.
+ * Checks if `value` is a Wilson prime.
  *
  * <https://en.wikipedia.org/wiki/Wilson_prime>
  *
  * @static
  * @memberOf primality
- * @param {Number} a
- * @returns {Boolean} Returns `true` if `a` is a Wilson prime.
+ * @param {Number} value
+ * @returns {Boolean} Returns `true` if `value` is a Wilson prime.
  * @example
  *
  * primality.isWilsonPrime(5);
  * // => true
  */
-function isWilsonPrime(a) {
-  return _.contains(WILSON_PRIMES, a);
+function isWilsonPrime(value) {
+  return _.contains(WILSON_PRIMES, value)
+         ? true
+         : 0 === (mod(factorial(value - 1) + 1, value) === 0);
 }
 
 /**
diff --git a/test/spec/suite.js b/test/spec/suite.js
index f004825..51b80c4 100644
--- a/test/spec/suite.js
+++ b/test/spec/suite.js
@@ -54,7 +54,7 @@ define(['../../dist/primality'], function(primality) {
         primality.isWilsonPrime(5).should.equal(true);
         primality.isWilsonPrime(13).should.equal(true);
         primality.isWilsonPrime(563).should.equal(true);
-        primality.isWilsonPrime(564).should.equal(false);
+        primality.isWilsonPrime(1000).should.equal(false);
       });
     });
   });