The microbe Mycoplasma genitalium has in several studies been proposed as an individual cause of non-gonococcal urethritis (NGU) in men, and has been associated with pelvic inflammatory disease (PID) and salpingitis. The prevalence of M genitalium has generally been 50-90% of the prevalence of C trachomatis, and this seems to be the case in Sweden as well. This is the first study of the pathogenesis and prevalence of M genitalium in northern Sweden. In total 823 samples, 340 from women and 483 from men, were screened for M genitalium by using a PCR method. Thirtythree (4.0%) patients, 13 (3.8%) women and 20 (4.1%) men, were infected by M genitalium. In the same group 60 (7.3%) patients, 16 (4.7%) women and 44 (9.1%) men, were infected by Chlamydia trachomatis. None of the 22 patients that were tested after treatment with azitromycin was still infected.