Previous studies of the treatment of nongonococcal urethritis (NGU) have generally not been closely controlled, and have often produced conflicting results. In the present study, 96 men with NGU were treated with placebo or tetracycline hydrochloride while "confined" aboard an aircraft carrier at sea. A seven-day tetracycline regimen was more effective than a four-day tetracycline regimen or placebo in the short-term cure of NGU.