Clinicopathological features of middle-aged lung cancer patients were investigated in 1016 consecutive patients. Among them, 22.8% of patients were in their forties and fifties. The preponderance of adenocarcinomas and the higher female/male ratio in middle-aged groups compared with the older group were findings similar to those observed in younger patients. Smoking habit increased according to the age groups. Middle-aged patients had a propensity for advanced stage, however, survival was not inferior to younger patients. Middle-aged patients consisted of two different groups of patients, whose characteristics were similar to those observed in either younger or elderly patients.