The aim of this study was to evaluate the prognosis for surgically treated young patients with primary lung cancer, a prognosis generally considered to be very poor. Eighty-two patients less than 40 years of age were operated on at Marie-Lannelongue Hospital between 1982 and 1990. There were 72 male and 10 female patients. Ten patients (12%) had never smoked, whereas 48 patients (59%) had smoked for more than 20 pack-years. The lung cancer was asymptomatic in 27 patients (33%) and symptomatic in the others. Adenocarcinoma was found in 42% of the patients, epidermoid carcinoma in 28%, mixed cell carcinoma in 16%, small cell carcinoma in 8.5%, and undifferentiated large cell carcinoma in 6%. Among the 69 resected tumors, 22 were stage I, ten were stage II, 32 were stage IIIa, and five were stage IIIb. The resection was considered complete and curative in 56 patients (68%) and noncurative in 26 (32%) either because of an incomplete resection (12 in stage IIIa; 1 in stage IIIb) or because of an exploratory thoracotomy only (13). The overall actuarial 5-year survival rate was 41%, and the actuarial 5-year survival for patients who had a complete resection was 56%. The actuarial 5-year survival rates were as follows: patients in stage I, 70%; stage II, 54%; stage IIIa, 28%; stage IIIb, 0%; and patients having exploratory thoracotomy only, 18%. These survival rates are similar to those of patients older than 40 years with similar stages of disease.(ABSTRACT TRUNCATED AT 250 WORDS)