Hidden field ... security issue

hi all,

   I have used a lots of hidden field in my web page to store file IDs etc .

I know that the user can easily find the ids in there hidden field . In every page before doing any operation I am cheking whether session is alive or not.

Now I want to know that what a melicious user can do with these hidden field values.

Can any one tell me some security issues linked with these.

Tnx and Regards