Seventy-eight men with a history of chronic urethritis were referred for investigation. Of 52 men diagnosed as having persistent or recurrent non-gonococcal urethritis (NGU) at the time of referral, 11 (21%) were infected with Mycoplasma genitalium and three with Chlamydia trachomatis. Men who were M. genitalium-positive had not previously received less antibiotic, in terms of treatment duration, than those who were M. genitalium-negative, suggesting a possible resistance to the antibiotics given. In the current investigation, of 11 M. genitalium-positive men with persistent or recurrent NGU who were treated for four to six weeks with erythromycin, 500 mg four times daily, nine (82%) responded clinically and microbiologically, but later six relapsed without M. genitalium being detected. The results of observing and investigating a patient for about one year, the only one to have concurrent chlamydial and mycoplasmal infections, is presented, a feature being the intermittent persistence of the mycoplasma.