diff --git a/src/math/Sphere.js b/src/math/Sphere.js
index f75fb2a17bf56913e420abbc7537e9042be32835..290963028ff2dffba7ead90748064ab3b5094bd4 100644
--- a/src/math/Sphere.js
+++ b/src/math/Sphere.js
@@ -67,9 +67,11 @@ THREE.Sphere.prototype = {
 
 	},
 
-	isIntersectionSphere: function(sphere) {
+	isIntersectionSphere: function( sphere ) {
 
-		return ( sphere.center.distanceToSquared( this.center ) <= ( this.radius * this.radius + sphere.radius * sphere.radius) );
+		var radiusSum = this.radius + sphere.radius;
+
+		return ( sphere.center.distanceToSquared( this.center ) <= ( radiusSum * radiusSum ) );
 		
 	},
 
diff --git a/test/unit/math/Sphere.js b/test/unit/math/Sphere.js
index 18a1f9d27c2633afbf47d328509efed4553d0074..baf057c0f29170e4e1ba66813b6abc7c6d91b9eb 100644
--- a/test/unit/math/Sphere.js
+++ b/test/unit/math/Sphere.js
@@ -60,6 +60,15 @@ test( "distanceToPoint", function() {
 	ok( a.distanceToPoint( one3 ) === -1, "Passed!" );
 });
 
+test( "isIntersectionSphere", function() {
+	var a = new THREE.Sphere( one3, 1 );
+	var b = new THREE.Sphere( zero3, 1 );
+	var c = new THREE.Sphere( zero3, 0.25 );
+
+	ok( a.isIntersectionSphere( b ) , "Passed!" );
+	ok( ! a.isIntersectionSphere( c ) , "Passed!" );
+});
+
 test( "clampPoint", function() {
 	var a = new THREE.Sphere( one3, 1 );