Summary. Bacteriospermia requiring medical treatment were diagnosed in more than 70% of the subfertile patients who had since 1988 attended the gynecological clinic at the RWTH hospital in Aachen. In 23% of all cases specific treatment with antibiotics did not reduce the concentrations of bacteria in sperma. Thirty-six patients with bacteriospermia resistant to antibiotic therapy were then subjected to dental examination. A high incidence of potential dental foci was found in all patients. In a test group of 18 patients these sources of potential infection were eliminated. Between dental operations and therapy swabs were taken to determine bacterial levels and bacteriological composition. It could be demonstrated that the bacterial spectrum of the intraoral samples was almost identical with the spermiograms. Six months following completion of dental treatment a further spermiogram analysis was carried out. In the test group about two thirds of the spermiograms proved sterile. Spermatological parameters, such as motility, density and morphology, had also clearly improved. In the control group the findings of the spermiogram remained poor. This study indicates that a direct causal relationship exists between bacterial colonies (dental foci) and therapy-resistant bacteriospermia which probably leads to subfertility.