An efficient authentication is made possible by a method for authenticating a subscriber MT (6) for utilizing services in a wireless LAN (WLAN) (10) while using an IP multimedia subsystem (IMS) (3). The inventive method is characterized in that a subscriber MT (6), who is to be authenticated and who is located at a location having WLAN coverage, receives an IP address from the WLAN (10) in an attributed manner, after which the subscriber authenticates himself with regard to the IP multimedia subsystem (3) while giving this IP address. In addition, an element ((WAGW (2)) of the WLAN (10) is informed of the result of the authentication of the subscriber MT (6) with regard to the IMS (3).