In the course of a controlled trial of bacillus of Calmette and Guerin vaccination among Puerto Rican children in 1949–1951, 82, 269 reactors to 1 or 10 tuberculin units of purified protein derivative were identified. During the 18 to 20 years after initial testing, 1400 cases of tuberculosis were identified among these tuberculin reactors. The major risk factor was age. Children under four years of age had the highest tuberculosis rates and the most serious disease. A secondary peak of incidence was observed at about 20 years of age. At all ages, children with the strongest sensitivity to tuberculin had the highest rates of subsequent tuberculosis. The risk for females and for urban residents was slightly greater than for males and rural residents. Essentially no difference in tuberculosis risk was found between white and black reactors. Because the risk of tuberculosis among infected persons appears to persist for a lifetime, the need for preventive treatment is highly dependent on age, and to a considerable but lesser extent on degree of tuberculin sensitivity.