The lung cancer in patients under 40 years old is rare and reported to be more rapidly fatal than in older persons. We reviewed the records of 18 cases who younger than 40 years with histologically proven lung cancer between 1959 and 1992 at department of Thoracic Surgery in National Medical Center. There were 12 male and 6 female patients. Two male and all female patients had never smoked. The 17 cases had respiratory symptoms for a mean duration of 3 months. The squamous carcinoma was found in 7 cases[38.9%], small cell carcinoma in 4 cases[22.2%], mixed cell carcinoma in 4 cases[22.2%], adenocarcinoma in 2 cases[11.1%] and bronchoalveolar cell carcinoma in 1 case. Among the 6 resected tumors, one case was stage I, two cases were stage II and three cases were stage IIIa. The operation was considered complete and curative in 6 patients and noncurative in 6 pations because of an exploratory thoracotomy. We conclude that lung cancer in young persons is virulent and that diagnosis is frequently delayed.