To assess the presence of Chlamydia trachomatis in nonacute abacterial prostatitis 30 patients with urethral cultures positive for Chlamydia trachomatis underwent microbiological studies, including cultures of transrectal aspiration biopsies of the prostate. Chlamydia trachomatis was isolated from 10 of the prostatic specimens (33 per cent). In 3 cases a nonspecific cytopathogenic effect caused the destruction of the tissue cultures. Our findings demonstrate that Chlamydia trachomatis may cause ascending infections of the prostate and that this microorganism may have an etiological role in the pathogenesis of nonacute abacterial prostatitis.