The association between acute epididymitis and development of antisperm antibodies was investigated by the gelatin agglutination technique in prospectively collected serum samples from 27 patients. Agglutinating antisperm antibodies, mainly of the IgG class, were detected in seven men (27 per cent), a significantly increased frequency compared to prevalences previously found among blood donors and men from infertile couples. The antibodies developed de novo in four patients (15 per cent). The increased incidence of positive sera and the kinetics of the antibody responses strongly indicate a causal link between acute epididymitis and autoimmunization against spermatozoa. In the majority of the patients the antibody titers were low, being of only theoretical importance in respect to later fertility.