sessions and browser back

Hi, I am trying to create restricted pages for my php/mysql/apache server with sessions and passing session varibales to other pages for validation. Eventually I am doinng session_start() at the begining and checking whether logged in user is authorised to use this page by a routine. Now problem is, if filled in data in the form is incorrect, the forms gives an error. But when I press browser BACK button to get the filled in form , the form seems to get reset with _no_ data. When I try without session_start() at the begining of form , things seem to behave normally. Does session_start() reset the form and I guess browser should have returned be by fiiled in page from cache ? Please help -Bibhas