Human immunodeficiency virus (HIV) infection is, to a great extent, a sexually transmitted disease (STD). Its diffusion among the heterosexual population is still limited. STD treatment centres are particularly well organized to watch this diffusion. At the STD centre of the Saint-Louis hospital, Paris, we conducted a 6-week prospective study concerning the systematic detection of HIV-1 infection in 240 consecutive female out-patients in 1988, and in 504 male out-patients in 1989. The results obtained were as follow: 5/240 women (2.1 percent) and 19/504 men (3.8 percent) were seropositive for HIV-1. Out of these 24 subjects, 15 did not know they were seropositive. Predictive factors for seropositivity were male homosexuality, addiction to heroin and, in women, drug addicts as sex partners. Altogether, 23 of the 24 seropositive subjects had the classical risk factors for HIV-1 infection. None of the 744 subjects in this study were HIV-2 seropositive, and only 1 out of 504 men was HTLV-1 seropositive. We conclude that the prevalence of HIV-1 infection was high in our centre, and this prompts us to suggest that the serological test should be proposed to all out-patients and that patient's education and preventive measures should be organized by STD centres, even though the infection is still limited to patients at a particularly high risk (drug addicts, homosexuals, country of origin).