This paper reports the 15-year results of a controlled clinical trial of BCG and vole bacillus vaccines in the prevention of TB. Among 54 239 participants, 583 cases of TB developed, of which 70% were of pulmonary TB and 15% of tuberculous pleural effusion without evidence of pulmonary TB. The annual incidence (per 1 000) of TB was 0.28 among the 13 598 BCG-vaccinated participants and 1.28 among the tuberculin-negative unvaccinated participants admitted concurrently (a 78% reduction); for vole bacillus vaccine (5 817 participants) the figures were 0.29 and 1.50 (an 81% reduction). Protection extended to all forms of TB (in particular, 10 cases of tuberculous meningitis or miliary pulmonary TB developed in unvaccinated, and none in vaccinated, participants) and was similar in the two sexes. It decreased gradually during the period but was still substantial (59% for BCG vaccine and 73% for vole bacillus vaccine) 10-15 years after vaccination; no definite conclusion can yet be reached on the level of protection after 15 years. The annual incidence of TB in those with strong positive reactions to tuberculin on entry was particularly high (3.75) in the first 2.5 years, reflecting active infections acquired before entry; those with weaker positive reactions on entry had a lower incidence throughout than the tuberculin-negative unvaccinated group, indicating that a degree of naturally acquired immunity (though less than that conferred by vaccination) was associated with the low-grade tuberculin sensitivity. The incidence of every form of TB decreased steeply during the 15 years, except for genito-urinary TB of which there were 3, 5, and 14 cases in successive 5-year periods, principally among initially tuberculin-positive participants. This report also presents the findings with respect to nontuberculous diseases, discusses the place of vole bacillus vaccination, and describes a method for assessing the applicability of mass BCG vaccination in a given epidemiological situation.