In order to determine whether tuberculin testing caused a booster effect in children vaccinated with Bacillus Calmette-Guérin (BCG) at birth, we studied forty 6-year-olds by repeat tuberculin testing 2 weeks later on the contralateral forearm. All children were healthy and had no known exposure to tuberculosis. None of the children had a history of mycobacteriosis other than tuberculosis. The mean induration was 2.3 +/- 1.8 mm for the first tuberculin reaction and 7.6 +/- 3.3 mm for the second tuberculin reaction (P less than 0.005). Four children had positive reactions (greater than or equal to 10 mm) to the first purified protein derivative test; 18 children were positive upon retesting. Eleven of these latter children had increases of at least 6 mm from reactions less than 10 mm to greater than or equal to 10 mm. The size of the BCG scar was significantly correlated to the size of both the first and second purified protein derivative reactions (P less than 0.01), suggesting that the increased reactivity upon retesting was a consequence of sensitization induced by BCG vaccination 6 years earlier. All children remained healthy after this study was completed. Retesting of tuberculin reactivity within 2 weeks in BCG-vaccinated children with reactions less than 10 mm will produce reactions greater than 10 mm in some healthy children who may not require antituberculosis treatment.