The first described pathogenic organisms that caused urethritis were Neisseria gonorrhoeae and Chlamydia trachomatis. The significance of detecting mycoplasma with genital swabs remained unclear for a long time. Culture can differentiate between Ureaplasma urealyticum and Mycoplasma hominis. After introduction of nuclear acid amplification, Mycoplasma genitalium was additionally detected, while gene analysis differentiates between Ureaplasma urealyticum and Ureaplasma parvum. Mycoplasma genitalium has become the third most frequent pathogen causing non-chlamydial, non-gonococcal urethritis (NCNGU); Ureaplasma urealyticum is less often isolated. Because urethritis caused by Mycoplasma genitalium does not always respond to tetracycline, it is advisable to begin therapy with a macrolide. Mycoplasma hominis is a cofactor for bacterial vaginosis and pelvic inflammatory disease (PID). During therapy with metronidazole, the colonization of this mycoplasma is decreased indirectly.