To study the prevalence of HIV and other sexually transmitted diseases (STDs), sexual behaviour and condom use among heterosexuals, 193 women and 157 men entered a longitudinal study. Participants were recruited through an STD clinic, and had had five or more heterosexual partners in the preceding 6 months but had no other AIDS risk factors. Participants were divided in groups with respect to private and/or commercial sexual partners. One hundred and thirty-six (71%) women worked as prostitutes and had, on average, 115 customers a month, and 99 (63%) men had visited, on the average, eight prostitutes in the past 4 months. One hundred and seventy-one (89%) women (114 prostitutes) had had, on average, four partners, and 112 (78%) men (of whom 64 had visited prostitutes) had had, on average, seven private partners in the past 4 months. Vaginal intercourse was often or always reported by all participants whilst anal contact was seldom reported. Non-high-risk sexual techniques (for example, oro-oral, masturbation) were practised more frequently with private than with commercial partners (P less than 0.001). Although no differences were found in the frequency of practising high-risk sexual techniques (for example, vaginal, ano-genital), differences were found for frequency of condom use. Condom use during vaginal intercourse was reported more frequently with commercial than with private partners (P less than 0.01). However, if the number of partners is taken into account, prostitutes had unprotected vaginal intercourse with an estimated average number of 160 partners in 4 months, which is far more than customers (seven) and men and women with private partners only (four).(ABSTRACT TRUNCATED AT 250 WORDS)