OBJECTIVES: To determine the importance of world region of birth as a risk factor for HIV-1 infection, the likelihood of having an HIV-1 infection diagnosed and the likelihood of having another coexisting acute sexually transmitted infection (STI) among attenders at genitourinary medicine clinics. SUBJECTS: Specimens from attenders having routine syphilis serology at 15 sexually transmitted disease clinics in England, Wales, and Northern Ireland participating in the unliked anonymous seroprevalence monitoring programme from 1994 to 1996. METHODS: Limited data were collected with specimens that were irreversibly unlinked from the source patients before testing for antibodies to HIV-1. Numbers of specimens, the prevalence of HIV-1, the proportions of infections clinically diagnosed, and the presence of coexisting acute STIs were analysed according to world region of birth, sexual orientation, and injecting drug use. RESULTS: Between 1994 and 1996, 173,075 specimens were collected; 16.9% were from people born outside the United Kingdom. Risk of being HIV-1 positive was significantly higher overall for both men and women born abroad, but this was not the case for those born in south Asia (India, Pakistan, and Bangladesh). Homosexual and bisexual males born abroad were almost twice as likely to be HIV infected as their counterparts born in the United Kingdom. However, homosexual and bisexual men born in the United Kingdom accounted for almost three quarters of the 1174 HIV-1 positive specimens detected. Among 158,728 non-drug injecting heterosexuals the highest prevalence was observed in specimens from those men (4.0%) and women (5.8%) born in sub-Saharan Africa. The 6991 heterosexual men and women born in other European countries were also more likely to provide HIV-1 positive specimens than UK born heterosexuals. However, 39% of the HIV-1 positive specimens in heterosexuals come from clinic attenders born in the United Kingdom. Heterosexual males were generally less likely to have their infection diagnosed than females. There were 182 attendances (mostly from London clinics) non-drug injecting heterosexual men and women who were infected with both HIV-1 and an acute sexually transmitted infection; only 12% of whom had had their HIV-1 infection diagnosed. CONCLUSION: Among most people attending genitourinary medicine clinics, being born abroad is associated with an increased likelihood for HIV-1 infection. HIV-1 infected heterosexuals, of whom 46% are people from sub-Saharan Africa, are unlikely to have their infection clinically diagnosed and thus are unable to obtain appropriate treatment. The presence of HIV-1 infected heterosexual men and women with acute STI represents a potential source of heterosexual HIV transmission both for those born in the United Kingdom and born abroad.