Nonbacterial prostatitis is often difficult to differentiate from other prostatic complaints and remains a vaguely characterized syndrome. Prostatic fluid inflammatory cells and elevated immunoglobulins raise the suspicion that this syndrome is caused by some undetected infection. Prostatic fluid antibodies against Chlamydia trachomatis, Ureaplasma urealyticum, staphylococcus, Staphylococcus faecalis, Bacteroides fragilis and Clostridium perfringens were measured in men with nonbacterial and bacterial prostatitis, and men without urinary symptoms by an enzyme-linked immunosorbent assay. Prostate specific antigen and prostatic acid phosphatase were measured in the prostatic fluid as indirect measures of secretory activity. Of 44 men with nonbacterial prostatitis 9 (20%) had detectable prostatic fluid antichlamydial antibody titers, compared with 3 of 25 control men (12%) and 2 of 13 (15%) with bacterial prostatitis--no evidence for a higher prevalence of prostatic fluid antichlamydial antibody in men with nonbacterial prostatitis. Prostatic antibodies to the other organisms were rarely detected. When compared with unaffected men the low levels of prostate specific antigen and prostatic acid phosphatase, and more alkaline prostatic fluid in men with bacterial and nonbacterial prostatitis suggest that secretory dysfunction accompanies the inflammation. These data show that none of the organisms studied caused the majority of the cases of nonbacterial prostatitis and that either an agent as yet unidentified or multiple agents may be involved in the etiology of nonbacterial prostatitis.