Cannot modify header...

Hey...
Hvordan kan jeg fikse den her?

Warning: Cannot modify header information - headers already sent by (output started at /usr/home/web/web26128/index.php:15) in /usr/home/web/web26128/usersys/login_check.php on line 18

Og linje 18 er:
header ("location: index.php");

thx...