output of session_start()

Hi, I use a script that sends header information to the browser. When I paste session_start() before the header() lines, the script stops working. I need the session, because the variables are stored in the session. I use no cookies, but trans-sid. Does session_start() send some information before the header() command? --------------- <? session_start(); header("Content-disposition: attachment; filename=$file"); header("Content-type: application/x-octetstream"); header("Expires: 0"); readfile($path."/".$file); ?> --------------- Stefan