The booster or enhancement effect of repeated tuberculin skin testing in Calmette-Guérin bacillus (BCG)-vaccinated young adults was studied in 208 first-year medical, nursing, and medical technology students in Santiago, Chile, where BCG vaccine is usually administered at birth and at 6 and 14 yr of age. Thirty-three students had no BCG scar, 62 had one scar, 71 had two scars, and 42 had three scars. The mean age for each group was 19 yr. All students were healthy and had no known exposure to tuberculosis or history of tuberculosis or other mycobacterioses. The size in millimeters of induration of the first tuberculin reaction (PPD1) was clearly correlated with the number of BCG scars: 2.3 ± 4.6 for no scars; 6.7 ± 6.7 for one scar; 10.9 ± 5.9 for two scars, and 13.2 ± 5.3 for three scars. The second tuberculin reaction (PPD2), performed 2 wk later on the contralateral forearm, showed a marked increase in reactivity. The increase in reaction size was most evident in students who had BCG scars but who were initially PPD negative (< 10 mm). Smaller increases were observed in students without BCG scars, and also in those who had BCG scars but who were initially tuberculin positive (⩾ 10 mm). The persistence of the booster effect was evaluated by performing PPD3 1 yr later. PPD1-negative students with BCG scars maintained the increased level of reactivity to PPD2 after 1 yr. An immunizing effect of tuberculin testing was suggested in 11 nonimmunized students who were initially PPD negative. Seven of these students became positive upon retesting with PPD4 2 wk after placing PPD3.