Many developed countries are faced with the problem of reorganizing their tuberculosis-control programme to bring it into line with modern conditions. The study reported was undertaken to provide guidelines for this reorganization. It was begun in the district of Kolín, Czechoslovakia, with a population of some 100 000, in 1961 and is still in progress. The paper covers the first 4 years of the study.In 1961 a thorough check-up was made on all persons registered as having active or inactive tuberculosis, or fibrotic lung lesions. In 1961 and 1963 a mass X-ray and tuberculin-testing campaign, with 95% coverage, was carried out for all persons over 14 years of age. All persons with active tuberculosis received adequate treatment. Persons registered as having tuberculosis or suspected tuberculosis were subjected to regular photofluorographic and bacteriological investigations. Newborn infants were given BCG vaccination, and persons aged 14 years and 19 years with negative tuberculin reactions were vaccinated.The prevalence of bacillary tuberculosis fell from 150 cases in 1960 to 91 in 1964, mainly owing to a decrease in the number of chronic cases. The incidence of bacillary tuberculosis detectable by direct smear microscopy, however, remained at about 25 cases throughout the period 1961-64. The risk of developing tuberculosis was found to be highest in persons with fibrotic lung lesions or inactive tuberculosis, and in men above 45 years of age and women above 65 with previously normal photofluorograms.It is concluded from the study that in developed countries priority should be given to adequate treatment of all persons with active tuberculosis, and to early diagnosis in persons consulting physicians and in the high-risk population groups.