In order to gain insight into the heterosexual spread of HIV infections in Amsterdam, a study was carried out in 1991 among persons attending an outpatient clinic for sexually transmitted diseases (STD) and among a group of prostitutes and prostitutes' clients. Out of 2362 persons attending the STD clinic, 2138 (90.5%) could ultimately be examined for presence of HIV antibodies. The HIV seroprevalence was 4.2% (90/2138); among homosexual males it was 22% (70/324), among heterosexual male intravenous drug users 12% (2/17) and among heterosexual males who never had used i.v. drugs 0.5% (5/997). The HIV seroprevalence among female intravenous drug users was 41% (12/29) and that among females who never had used i.v. drugs it was 0.1% (1/771). Among the heterosexually active subjects (active in the preceding six months, including the groups of bisexual males and intravenous drug users), the seroprevalence amounted to 1.5% (28/1884). HIV antibodies could be demonstrated in 1.5% (3/199) of the prostitutes and in 0.5% (1/213) of prostitutes' clients. However, the one positive male had had homosexual contacts in the past. Out of the total of nine heterosexually infected persons, two originated from the Netherlands, two from Ghana, two from Turkey, one from Nigeria, one from Pakistan and one from the Dominican Republic; six of them had commercial contacts. The heterosexual spread of HIV among the heterosexual population of Amsterdam can for the moment be classified as slight. However, further heterosexual transmission of HIV cannot be excluded, considering the frequency of unprotected sex.