To test the hypothesis that the reduction in tar and nicotine content of cigarette smoke that began in the 1950s should be reflected by the histologic changes in the bronchial epithelium of cigarette smokers, 20, 424 sections taken at autopsy from the bronchial tubes of 445 men (non-lung-cancer deaths) were examined microscopically in random order. There were 211 men who died in 1955-1960, of whom 154 smoked regularly, and 234 men who died in 1970-1977, of whom 181 were regular smokers. Changes studied included basal-cell hyperplasia, loss of cilia and occurrence of cells with atypical nuclei. In both periods studied these histologic changes occurred far less frequently in nonsmokers than in cigarette smokers and increased in frequency with amount of smoking, adjusted for age. Sections with advanced histologic changes in those dying in 1955-1960 occurred in 0 per cent of nonsmokers, in 2.6 per cent of those smoking one to 19 cigarettes a day, in 13.2 per cent of those smoking 20 to 39 and in 22.5 per cent of those smoking 40+ cigarettes a day. In those who died in 1970-1977 the percentages were 0, 0.1, 0.8, and 2.2, respectively.