PROBLEM CLOSING A USER FORM

I have a user form which has a couple of text boxes. On exit of these boxes, my macro goes through a validation routine. The problem I have is that if the "user" clicks on a "Cancel" button before entering any data then the macro still drops in the exit routine. How can I get it to drop first in to the Cancel routine or either to test to see if the user has clicked on the cancel button. The start of my macro is:

"Private Sub TxtLia01_Exit(ByVal Cancel As MSForms.ReturnBoolean)"

but whatever I do Cancel is always equal to "false".