Message Box

Hey everyone,
Got a quick question for you all. I'm trying to go a form validation by using the onsubmit event. The onsubmit event works fine and a little msgbox appears when they do not select something in the drop down boxes, but the problem is that the msgbox re-appears after you hit "ok" the first time. Anybody have any idea why?

Here is my code:

<script language="vbscript">
dim validation

Function MyForm_OnSubmit
validation = true
If (Document.myForm.Num.Value) = "" Then
MsgBox "Please select a document number."
validation = False
End If

If (Document.myForm.Originator.Value) = "" Then
MsgBox "Please select an originator."
validation = False
End If

If validation = True Then
MyForm_OnSubmit = True

Else

MyForm_OnSubmit = False

End If

End Function


</script>

<form action="addNew.asp" name="myForm" method="post" onSubmit="MyForm_OnSubmit">