While the prevalence of HIV-1 is increased in women attending sexually transmitted diseases (STD) clinics, in South Africa little is known about STD risk factors in women infected with HIV-1 in the general population. Serological markers of STDs were investigated in a cohort of 21 pregnant women, who were found to be carriers of HIV-1, and in 42 age- and residentially matched uninfected pregnant women. The rapid plasma reagin test (RPR) was positive in 33% of the HIV-1- infected women (P less than 0.05 compared with controls). The specific treponemal fluorescent antibody absorbed test (FTA-abs) was positive in 43% of the HIV-1-infected women (P less than 0.05 compared with controls). Similarly the prevalence of chlamydial IgG antibodies as measured by enzyme-linked immunosorbent assay was 95% (P = 0.026 compared with controls). Markers of hepatitis B infection were similar to both groups. Pregnant women with HIV-1 infection in the Johannesburg area have an increased seroprevalence of markers of STDs and more data are required on the attitudes and sexual activity of these women in order to target AIDS and education information.