Most cases are caused by either Neisseria gonorrhoeae or Chlamydia trachomatis. Other causative organisms include Mycoplasma genitalium, Ureaplasma urealyticum, Trichomonas vaginalis, and HSV. Chlamydia causes 30-40% of nongonococcal urethritis (NGU) cases. M. genitalium is the probable cause in many Chlamydia-negative cases of NGU.