Two hundred and thirty-three men and 85 women visiting STD clinics in western Sweden between April 1997 and March 1998 were examined for Mycoplasma genitalium and Chlamydia trachomatis. The bacteria were identified by the polymerase chain reaction (PCR) technique. Three women (3.5%) and 18 men (7%) were positive for M. genitalium. Seventeen (14%) of the 115 men with urethritis were infected but only one of the men was without urethritis. After treatment with tetracyclines for 10 days, one woman and 8 of the 13 men still harboured M. genitalium. M. genitalium-infected men did not have more life-time partners than other men visiting STD clinics. More men positive for M. genitalium gave a history of previous urethritis but the difference was not significant.