Cigarette-smoking males (6,364), aged 40-64, were randomized into an intervention group which received 6-monthly screening by chest X-ray and sputum cytology, and a control group which received no asymptomatic investigation. After 3 years, both groups entered a follow-up period during which they received annual chest X-rays. Lung cancer cases detected by screening were identified at an earlier stage, more often resectable, and had a significantly better survival than "interval" cases diagnosed mainly because of symptoms. Comparison of the 2 groups showed a higher incidence of lung cancer in the intervention group, despite the follow-up period when both groups received annual examinations. There was no significant difference in mortality between the 2 groups.