To elucidate the role of Ureaplasma urealyticum as a causative organism in chronic prostatitis we investigated its incidence in the urogenital tract in 131 chronic prostatitis patients and 120 prostatodynia patients; the presence of common bacteria or Chlamydia trachomatis was also recorded. According to the 4-glass localisation test, U. urealyticum (greater than or equal to 10(3) ccu/ml) was isolated from the prostates of 16 prostatitis patients and 2 prostatodynia patients; 5 of these prostatitis patients (but neither of the prostatodynia patients) had other bacteria in specimens after prostatic massage. Thus U. urealyticum was the sole organism isolated from the prostates of 11 prostatitis and 2 prostatodynia patients, which suggests that it is an aetiological agent in some cases of chronic non-bacterial prostatitis.