sessions don't work

Hello, I have a problem with php sessions. I have a php file like: <? session_start(); session_register('pagina'); ?> <html> <head> ... </head> <body> <? include('file'); ?> ... </html> I have done diferent proofs and I have seen that the problem is that when I put include don't work sessions and if I left session it works. When I put all together it prints in the browser the code from "<html>" to "<body". It don't continue. If I reload the page it works properly. But I haven't found the way to do it automaticaly. Are there somebody who knows how to fix it? Thanks. Un saludo.