In 17 out of 102 patients with clinically diagnosed chronic prostatitis the disease could be attributed to known urogenital tract pathogens. Of the remaining 85 patients, Ureaplasma urealyticum was isolated from 38, Chlamydia trachomatis from five, and both organisms together from two. The results of antimicrobial treatment of the patients suggest an etiological relationship between Ureaplasma urealyticum and certain cases of chronic prostatitis. In these cases urethritis seems to be an accompanying symptom (urethro-prostatitis). No relation could be demonstrated between a favorable outcome of therapy and particular serotypes of ureaplasma. Our study could not establish any pathogenic role for Mycoplasma hominis.