The belief that HIV-1 infection causes AIDs has been questioned, and the suggestion made that to know the correct cause of AIDS the incidence of disease in patients with and without risk behaviours and with and without antibody to HIV-1 must be known. We describe findings in such a cohort. In 715 homosexual men followed for a median of 8.6 years, all 136 AIDS cases occurred in the 365 individuals with pre-existing HIV-1 antibody. Most men negative for HIV-1 antibody reported risk behaviours but none developed any AIDS illnesses. CD4 counts fell in anti-HIV-1-positive men but remained stable in antibody-negative men, whether or not risk behaviours were present. The hypothesis that AIDS in homosexual men is caused not by HIV-1 infection but by drugs and sexual activity is rejected by these data. HIV-1 has an integral role in the pathogenesis of AIDS.