diff --git src/css.js src/css.js
index 30cecf3..878dd66 100644
--- src/css.js
+++ src/css.js
@@ -6,6 +6,7 @@ var ralpha = /alpha\([^)]*\)/i,
 	rupper = /([A-Z])/g,
 	rnumpx = /^-?\d+(?:px)?$/i,
 	rnum = /^-?\d/,
+	rrelnum = /^([+\-]=)?([\d+.\-]+)(.*)$/,
 
 	cssShow = { position: "absolute", visibility: "hidden", display: "block" },
 	cssWidth = [ "Left", "Right" ],
@@ -91,6 +92,14 @@ jQuery.extend({
 				value += "px";
 			}
 
+			if ( typeof value === "string" ) {
+				var parts = rrelnum.exec( value );
+				if ( parts && parts[1] ) {
+					var relVal = parseFloat( parts[2] );
+					value = ( ( parts[1] === "-=" ? -1 : 1 ) * relVal ) + jQuery.css( elem, name );
+				}
+			}
+
 			// If a hook was provided, use that value, otherwise just set the specified value
 			if ( !hooks || !("set" in hooks) || (value = hooks.set( elem, value )) !== undefined ) {
 				// Wrapped to prevent IE from throwing errors when 'invalid' values are provided
diff --git test/unit/css.js test/unit/css.js
index 4ec7c60..c3fcae5 100644
--- test/unit/css.js
+++ test/unit/css.js
@@ -103,6 +103,22 @@ test("css(String|Hash)", function() {
 	equals( child[0].style.fontSize, old, "Make sure font-size isn't changed on null." );
 });
 
+test("css(String, String|Object) with relative values", function() {
+	var $elem = jQuery('#nothiddendiv');
+	$elem.css({ width: 1, height: 1 });
+	ok( [$elem.width(), $elem.height()], [1,1] );
+	$elem.css({ width: "+=9", height: "+=9" });
+	ok( [$elem.width(), $elem.height()], [10,10] );
+	$elem.css({ width: "-=9", height: "-=9" });
+	ok( [$elem.width(), $elem.height()], [1,1] );
+	$elem.css({ width: "+=9px", height: "+=9px" });
+	ok( [$elem.width(), $elem.height()], [10,10] );
+	$elem.css({ width: "-=9px", height: "-=9px" });
+	ok( [$elem.width(), $elem.height()], [1,1] );
+	$elem.css("width", "+=9").css("height", "+=9");
+	ok( [$elem.width(), $elem.height()], [10,10] );
+});
+
 test("css(String, Object)", function() {
 	expect(22);