A study of 304 sexually active homosexual men, most of whom had multiple casual partners, showed that receptive anogenital intercourse, independent of anal bleeding, was the only risk factor for HTLV-III/LAV transmission. There was no evidence that HTLV-III/LAV infection, measured by seropositivity, was transmitted by oroanal or orogenital routes, or that insertive penile intercourse constituted a risk. The strongest predictor of seropositivity proved to be homosexual activity for more than five years, which may lead to enhanced susceptibility to infection. Sexual exposure to European men seemed to be even more hazardous than sexual exposure to men from the United States of America, and emphasised the epidemiological importance of the promiscuous homosexual abroad. Skin complaints were the most common presenting symptoms in men with antibody to HTLV-III/LAV. Extrainguinal lymphadenopathy was the commonest sign, which was present in just under half of those who were seropositive. More than one quarter of seropositive patients had had sexual intercourse with a woman in the five years before being tested.