A study of risk factors for HIV-1 infection was conducted at a sexually transmitted disease clinic in an area of New York City where the cumulative incidence of AIDS in adults through mid-1990 was 9.1 per 1000 of the population and where the use of illicit drugs, including smokable freebase cocaine (crack), is common. The overall seroprevalence among volunteers was 12% (369 out of 3084), with 80% of those who were seropositive reporting risk behavior associated with HIV-1 infection, including male-to-male sexual contact, intravenous drug use and heterosexual contact with an intravenous drug user. The seroprevalence in individuals denying these risks was 3.6% (50 out of 1389) and 4.2% (22 out of 522) in men and women, respectively. Among these individuals, the behaviors significantly associated with infection were use of crack and prostitution in women, and history of syphilis and crack use in men. These results suggest that in areas where the level of HIV-1 infection in heterosexual intravenous drug users is high and the use of crack is common, increased sexual activity (including the exchange of drugs or money for sex) may result in increased heterosexual transmission of HIV-1.