Very simple session question

Hi I've gone bozzeyed, so I've a simple question for you. I open session_start() as the first thing, on each page. I set a session variable using, for instance, $_SESSION['ACCOUNTemail'] = $email; The next program, when I var_dump($_SESSION), it shows an empty array. That program does run session_start() too. What's obviously wrong, or what's my most likely cause? Cheers J