A randomized prospective study of lung cancer detection was begun in 1976 to evaluate semiannual screening by radiologic and sputum cytologic study in comparison to screening at a 3-year interval, and to no screening. In a high-risk population of 6364 men (aged 40 to 64 years), the initial prevalence of lung cancer was 0.28% (18 cases), the annual incidence was 0.35% per year (66 cases during 3 years), the proportion of Stage I cases was 31% (26/84), and Stage II was 17% (14/84), "curative" resections were 27% (23/84), and 5-year survival was 23% (19/84). The study confirmed the ability of radiologic screening to detect lung cancer at an earlier stage when treatment by resection can be accomplished. The fate of a high-risk population submitted to screening was better than that of a population with no screening where lung cancer was discovered by symptoms, accidental x-rays, or at autopsy. A matter of lesser importance was the frequency of screening. The absolute numbers of 5-year survivors detected by screening were practically the same for either compared screening frequency.