The aetiological importance of Chlamydia (C.) trachomatis in non-gonococcal urethritis (NGU) is undisputed. Mycoplasma (M.) genitalium has been shown to be strongly associated with NGU and with mucopurulent cervicitis and also with acute endometritis independent of C. trachomatis. In this prevalence study we examined 946 patients, 445 women and 501 men, attending the STD clinic, for M. genitalium as well as C. trachomatis and Neisseria gonorrhoeae. M. genitalium was detected in urethral samples from 17 (13.4 per cent) of 127 men with both symptoms and signs of urethritis and from 2 (1.3 per cent) of 160 men without (p < 0.001). Corresponding figures for M. genitalium in the women were 15 (11 per cent) of 136 women with symptoms and signs of urethritis or cervicitis compared to 3 (2.2 per cent) of 139 women without (p = 0.005). Examinations of partners of female and male index patients indicated that M. genitalium is sexually transmitted. Some M. genitalium infected patients had a history of irregular vaginal bleeding, lower genital tract pain, epididymitis and arthritis. Investigation of the aetiological role of M. genitalium in salpingitis, epididymitis and sexually acquired arthritis is urgently needed.