From 1949 to 1951, a total of 191,827 children in Puerto Rico were enrolled in a controlled trial of BCG vaccination. Of these children, 1 through 18 years of age, 82,269 were classified as reactors to tuberculin and 109,558 as nonreactors. Of the nonreactors, 31,856 refused vaccination, 27,338 were left unvaccinated as controls, and 50,674 were vaccinated with BCG. By the end of June 1969, a total of 37 cases of cancer had been diagnosed among the controls and 98 among the "vaccinees," yielding average annual rates of 7.2 and 10.3 cases per 100,000 population, respectively. The vaccinated group had a slight deficiency of leukemia cases and an excess of lymphosarcoma and Hodgkin's disease. The excess risk of cancer was concentrated among children age 10 through 18 on entry into the trial.