hi im tryin to use the return from a jquery ajax call in my own function, but it keeps returning undefined.

    function checkUser2(asdf) {
    $.ajax({
        type: "POST",
        async: false,
        url: "check_user.php",
        data: { name: asdf },
        success: function(data){ 
            return data;
            //alert(data);
        }
    });  
}       


$("#check").click(function(){
    alert(checkUser2("muma"));
});

the ajax call definately works, because when i uncomment my alert i get the correct return, and i can see it in firebug. Am i doing something stupid.