To assess the potential role of drug using prostitutes in the heterosexual transmission of human immunodeficiency virus (HIV), 117 prostitutes, recruited from December 1985 to November 1987 at a weekly evening sexually transmitted diseases (STD) clinic for drug using prostitutes only, were studied for the prevalence of HIV and STD. Intravenous drug use was reported by 96 (82%) of the women. Antibody to HIV was found in 35/117 (30%) women, all but one of whom had been using drugs intravenously. Most (73, 62%) of the prostitutes reported having had more than 2000 clients each in the preceding five years. In the six months preceding enrollment to the study they had practised mainly vaginal (106/117) and orogenital (82/117) intercourse; 90% (105/117) reported frequent use of condoms in vaginal intercourse and 65% (68/105) in orogenital intercourse. Despite frequent condom use, 81% (84/104) contracted one or more STD in this period. Considering the high incidence of HIV and STD and the finding by others that the presence of STD may facilitate transmission of HIV, we conclude that the potential for HIV transmission from these prostitutes to their clients (and vice versa) was clearly present.