In order to define the roles of the K-ras and p53 genes in the development of lung cancer, especially in young adults, we compared the clinicopathological features of the patients between younger (< or = 45 years, n = 47) and older (< 55 years, n = 50) groups. The gene alterations were examined by the polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) method. The K-ras gene alterations were detected only in adenocarcinomas, and the p53 gene alterations in all histologic types of lung cancer. There were no significant differences in the frequency of both K-ras and p53 gene alterations between the younger and older groups (9 vs. 11%, 36 vs. 32%). In the younger group, but not in the older one, the percentage for smokers was significantly higher in the p53 gene alteration-positive group than for the negative group (65 vs. 30%). As to the prognosis, there were no significant differences between the p53 gene alteration-positive and -negative cases in both the younger and older groups as well as in all subjects, while a tendency of poorer prognosis was observed in K-ras gene alteration-positive cases than for the -negative ones with adenocarcinomas. These results suggest that (1) the K-ras and p53 gene alterations would have no special roles in terms of the lung carcinogenesis in young adults; (2) a positive relationship between smoking and p53 gene alteration would exist in young adults with lung cancer, and (3) K-ras gene alteration would become a prognostic factor in lung cancer.