Besides Chlamydia trachomatis, various microorganisms could cause non-gonococcal urethritis (NGU). Recently, Mycoplasma genitalium and Ureaplasma urealyticum (biovar 2) have been suggested to be other pathogens of NGU independent of C. trachomatis. Clinical findings of non-chlamydial NGU, including M. genitalium--or U. urealyticum-postive NGU, are not different from those of chlamydial NGU. M. genitalium and U. urealyticum (biovar 2) are susceptible to tetracyclines, macrolides, and fluoroquinolones. However, the post-treatment presence of M. genitalium in the urethra is significantly associated with persistent or recurrent urethritis. Eradication of this mycoplasma from the urethra is essential for managing M. genitalium-positive NGU. In treatment of non-chlamydial NGU, therefore, the antimicrobial agents that are active against M. genitalium should be chosen.