An abrupt decrease of the number of reported new AIDS cases took place in the first half of 1997 in the Netherlands. This decrease was most prominent in homosexual/bisexual men and intravenous drug users, and less conspicuous in heterosexual men and women. The number of heterosexual patients with a non-Dutch nationality even increased compared with the first half of 1996. The most probable explanation of these observations is the introduction of improved treatment possibilities for HIV-infected people starting July 1996, which lead to a longer incubation period. Owing to these developments monitoring the HIV/AIDS epidemic in the Netherlands will depend more and more on HIV status surveillance instead of on registration of new AIDS cases.