To assess the possible role of sexually transmitted diseases as cofactors for the spread of AIDS, 248 adult patients were tested for the presence of antibodies against human immunodeficiency virus (HIV), hepatitis B virus (HBV) and syphilis. The survey was conducted in a hospital at Kagondo, Kagera Region, Northwest Tanzania, Africa. Subjects were randomly chosen from the outpatient clinic to include those with and without sexually transmitted diseases, as well as AIDS/pre-AIDS patients. The data were univariately and multivariately analysed by linear logit models, including interactions of demographic parameters. The results obtained reveal a strong association between the presence of antibodies against HIV, syphilis and HBV, respectively. The HBV/HIV-correlation remained stable in the multivariate analysis including interactions of social parameters, in contrast to the syphilis/HIV-correlation. We assume that this reflects the lower virulence of HBV as compared to that of syphilis. The prevalence of anti-HBV antibodies seems to be a more reliable marker for high sexual activity than those against syphilis. The possibility that HBV and HIV act as cofactors for each other's transmission could not be ruled out.