146 men with rheumatoid factor-negative (sero-negative) arthritis, i.e., 97 patients with ankylosing spondylitis, 36 patients with Reiter's syndrome, and 13 patients with reactive arthritis, were examined for infections of the urogenital tract by following recently established criteria. 74 patients (50.7%) had infections of the male adnexes: 3 patients suffered from balanitis, 14 patients from urethritis, 49 patients from prostatitis, 1 patient from epididymitis, and 7 patients from urinary tract infection. Balanitis and urethritis were almost exclusively associated with Reiter's syndrome. In 37 of 97 patients with ankylosing spondylitis, a urogenital tract infection, mainly a prostatitis (31 patients), was detected. The microorganisms isolated most frequently from patients suffering from urethritis and prostatitis, were Chlamydia trachomatis and Ureaplasma urealyticum.