The Memorial Sloan-Kettering Lung Cancer Detection Program is described, and data from this study are reported and analyzed. The program enrolled 10,040 cigarette-smoking men who were screened for at least 5 years by annual chest radiographs; half the men randomly selected also had 4-monthly sputum cytologic examinations. Of the 354 lung cancers that developed in these men during the screening period and a 2-year post-screening period, nearly half were adenocarcinomas and nearly one third were epidermoid (or squamous) carcinomas. Lung cancers detected by screening were more likely to be early stage (53 per cent) and resectable (65 per cent) than were those discovered by other means (for example, symptoms) or in the post-screening period (20 per cent early stage, 32 per cent resectable). Sputum cytology was most effective in detecting squamous carcinomas early; chest radiographs were most effective for detecting adenocarcinoma. However, sputum cytology did not reduce lung cancer mortality among these men who were in a program of annual chest radiographic examinations. Overall 5-year survival for all of the men who developed lung cancer was 35 per cent, compared with the national average of 13 per cent.