A one-year study of an outbreak of pulmonary tuberculosis was conducted aboard a ship with 1,175 men. Twenty-five cases of active pulmonary tuberculosis were discovered. Purified protein derivative of tuberculin, Seibert (PPD-S) skin tests were performed serially, and roentgenographic studies were performed at two-month intervals on 532 men chosen because of their close proximity to the three initial active cases of pulmonary tuberculosis. The rest of the ship's complement was studied similarly twice during the year. Tuberculin reactivity rate went from 14.3% to 53.6% among 532 men. All PPD-S skin test converters underwent complete clinical evaluations. Airborne infection is suggested by the rapid spread and extraordinarily high rate of tuberculin conversion. Particular consideration is given to the high susceptibility of tuberculin-sensitive young adult men. Studies of all contacts at frequent intervals are reemphasized as the best means of preventing the spread of pulmonary tuberculosis in a closed environment.