The criteria that need to be fulfilled before regarding a mycoplasma as a cause of non-gonococcal urethritis (NGU) are outlined. Of the seven mycoplasmas that have been isolated from the human genitourinary tract, most cannot be considered as contenders for causing NGU. Although there is no evidence to support an etiological role for Mycoplasma hominis, it may be unwise to ignore this mycoplasma in view of its known pathogenicity in other situations. The cumulative weight of evidence indicates that strains of Ureaplasma urealyticum (ureaplasmas) cause NGU in some patients. The reason for their occurrence in the urethra of some men without disease needs to be established. Ureaplasmas do not seem to cause post-gonococcal urethritis. The role in NGU of M. genitalium, newly discovered in the male urethra, is unknown, but its biological features, morphological appearance, and ability to cause genital disease in marmosets suggest that it may be pathogenic for man.