Objective To assess trends in HIV infection among sexually transmitted disease (STD) patients. Design Repeated unlinked anonymous survey, 1991–1993. Setting STD clinics in Paris, France. Subjects Patients (n = 4354) with a new suspected STD. Methods HIV antibody testing, using blood from syphilis samples. Results HIV prevalences were stable over time in all transmission groups. One-third of homo-/bisexual men were HIV-positive. Prevalence was 2.5 times higher among heterosexual patients from Africa or the Caribbean than among those from other countries. Among patients under 25 years of age prevalence significantly decreased from 4.3% in 1991 to 0.8%, in 1993 (P= 0.01). Among homo-/bisexual men, despite a 50% reduction in the incidence of STD, the absolute number of those newly HIV-infected remained stable; median age increased from 28 years in 1991 to 32 years in 1993 (P=0.02). Among heterosexuals, trends in HIV incidence were difficult to assess: recently infected patients were more likely to be identified in 1993 than in 1991, since the proportion of patients who reported a recent HIV-negative test increased over time. Conclusion Prevalence studies contribute to define specific subgroups which should be targeted for prevention (HIV-positive or older homosexuals, heterosexuals from Africa and the Caribbean). Despite a decrease in both overall STD incidence and HIV prevalence among patients aged under 25 years, overall HIV incidence has not decreased, at least among homo-/bisexual men in whom recent HIV infections occurred at a high rate overall, and increased in those aged 35 years or more. Sentinel site-based HIV seroprevalence studies are best interpreted in the light of results obtained from different populations and through routine surveillance of STD.