Objective: This survey was conducted to determine the prevalence of HIV infection and associated risk behaviour in a high risk population of clients attending an STD clinic in Prague, Czech Republic. Methods: Between September 1994 and January 1995 clients entering the Apolinar STD clinic in Prague, Czech Republic, were enrolled in a blinded, unlinked HIV antibody seroprevalence study. Non-identifying demographic characteristics, STD diagnoses, HIV risk exposures, and voluntary HIV testing experience were extracted from medical charts. Results: Of 1394 patients for whom serum was available for testing, one was positive for HIV (HIV prevalence 0.07%, 95% confidence interval (CI) 0.01, 0.41%). This person was one of 28 men having sex with men (MSWM) (HIV prevalence among MSWM 3.6%, 95% CI 0.6, 17.7%). Among the 775 male clients, 75.5% had heterosexual unprotected sex, 11.1% had sex with high risk partners or prostitutes, 3.6% were MSWM, 1.0% were injecting drug users (IDUs), 0.7% were both MSWM and IDUs, and 6.8% and 1.8% had other or no recognised risk for HIV/STDs, respectively. Among the 619 female clients, 74.5% had heterosexual unprotected sex, 11.6% were prostitutes, 7.8% had sex with high risk partners, 1.1% were IDUs, and 3.9% and 2.3% had other or no recognised risk, respectively. The 304 adolescent patients (age 11-19 years) diVered significantly (p<0.05) in risk behaviour and STD diagnoses from the 1090 patients who were 20 years and older. Adolescents were significantly more likely to be female (58.6% v 40.5%, OR=2