Unity 4: GUI.Button does not work anymore?

Hey guys! I'm trying out the new Unity 4.0 and have a big problem with GUI.Button. This method worked well in Version 3.5 but no buttons appear if I try this in Unity 4.0. I created a new empty GameObject and wrote the script below. After saving the script I attached it to the GameObject.

     float buttonX; 
     float buttonY;
     float buttonWidth;
     float buttonHeight;
     
     // Use this for initialization
     void Start () {
         buttonX = Screen.width * 0.05f;
         buttonY = Screen.width * 0.05f;
         buttonWidth = Screen.width * 0.1f;
         buttonHeight = Screen.width * 0.1f;
     }
     
     //Draw GUI
     void onGUI(){
         GUI.Button (new Rect(buttonX, buttonY, buttonWidth, buttonHeight), "Start");
     }

Maybe I am wrong but I think this should work. Am I doing it wrong or is it something new in Unity 4.0?

Thanks a lot!

mischaal