We studied the clinical characteristics and prognosis of patients, 40 years old or younger, in whom primary lung cancer was diagnosed and treated at National Nishigunma Hospital between 1982 and 1993, and compared them with those of 978 patients more than 40 years old. Younger patients numbered 30 (3.0%). Females accounted for 16 of the 30 cases (53.3%), a proportion higher than the female: male ratio for the older patients (27.8%). There were more smokers among the older patients (72.8%) than among the younger patients (53.3%) (p < 0.01). Adenocarcinoma was significantly more common (19/30, 63.3% vs 43.8%, p < 0.05) and squamous cell carcinoma was less common (3.30, 10.0% vs 34.3%, p < 0.05) in the younger patients than in the older patients. Median survival time in younger patients was 30.0 months, and in older patients it was 14.6 months, but we found no significant difference in survival between younger and older patients. In the younger group, all the cases of stage I or II disease were discovered during mass screening.