The effect on tuberculin reactivity of bacille Calmette-Guérin vaccination (BCG-V) given 10 to 25 yr earlier was measured among schoolchildren and young adults in Montreal. Of a total eligible population of 8,264 persons, 5,952 were tuberculin tested (72%). The BCG-V status could be verified for 4,629 of 4,969 (93%) participants born in Quebec. Of these, 1,511 (33%) had received BCG-V, 66% of whom had been vaccinated once only in infancy, 23% of whom had been vaccinated once but after infancy, and 11% of whom had been vaccinated twice. Among those vaccinated in infancy, 7.9% had significant tuberculin reactions, compared with 18% among those vaccinated between 1 and 5 yr of age, and 25.4% among those vaccinated after the age of 5 (p less than 0.001). Among those receiving BCG-V in infancy, the prevalence of tuberculin reactions of 10 mm was similar to the nonvaccinated subjects, after accounting for differences in age and socioeconomic status. Among those vaccinated after infancy, tuberculin reactivity was not significantly affected by interval from BCG-V until tuberculin testing nor by repeat BCG-V. We conclude that after an interval of 10 to 25 yr the most important determinant of effect of BCG-V on tuberculin reactivity is age when vaccinated. A significant tuberculin reaction among those who have received BCG-V should be interpreted in light of the age when vaccinated and the expected prevalence of tuberculous infection in the population tested.