The effectiveness of BCG vaccination, at about age 13 years, in the prevention of tuberculosis at ages 15-24 years in England and Wales in 1983, has been determined by the same method as in two previous surveys in 1973 and 1978. In 1983, the tuberculosis notification rate among those vaccinated in the schools' scheme was 3.30 per 100,000, compared with an estimated rate of 13.20 per 100,000 among those who were tuberculin negative and not vaccinated in the scheme. The protective effectiveness of BCG vaccination at ages 15-24 years in England and Wales in 1983 is thus estimated as 75%; the estimated efficacy in the white ethnic group is closely similar, namely 76%. The estimated efficacy of BCG at ages 15-19 and 20-24 years in the three surveys, both in the white ethnic group and in the entire cohort, has been compared with that found in the Medical Research Council's controlled trial of tuberculosis vaccines which began in 1950. There is no evidence of any decrease in the protective efficacy of BCG vaccination between the four cohorts of young adults, which span a total period of 29 years. However, there were steep decreases between the cohorts in the annual notification rates for the white ethnic group; these decreases occurred in the BCG vaccinated and in the tuberculin negative unvaccinated groups, as well as among those found tuberculin positive (and not vaccinated) in the schools' scheme. It is concluded that the level of protective efficacy of BCG vaccination at ages 15-24 years is high, and has remained unchanged since the start of the BCG in schools' scheme. However, as the tuberculosis notification rate in young adults has decreased steeply throughout this period, and is continuing to decrease, the benefits to be expected from the BCG in schools' scheme will decrease equally rapidly.