We studied 187 men with symptoms and signs of chronic prostatitis or prostatosis and 108 healthy controls. Ureaplasma urealyticum was isolated from 103 (55.1 per cent) of the patients and from 24 (22.2 per cent) of the controls. When quantitative culture of Ureaplasma urealyticum is considered a typical histogram of prostatitis was evident in 36 patients (19.3 per cent) using the Stamey localization technique. In 20 of the 36 patients the etiology was uncertain for various reasons. Evidence was provided that Ureaplasma urealyticum could be considered as the etiologic agent of the disease in at least 16 (8.6 per cent) of the 187 patients with chronic prostatitis studied.