PHP 4 session's probelm/design issue ?

Hi folks, Here is the issue I encountered using the session feature in PHP 4.02pl2 : I have a blank form allowing user to enter data. Upon submission, if user did not key in correctly, I direct the user to an error screen. The issue started here --> when the user clicks the "back" button, all of the previous entered data were gone. If I took out the session things, this won't happen. How could I solve this ? Can someone help, please ? Thanks, YY