The three well-characterized genes of the ras gene family H-ras, K-ras, and N-ras, code for closely related 21-kD proteins that have a role in the transduction of growth signals. The ras proteins acquire transforming potential when a point mutation in the gene leads to replacement of an amino acid in one of the critical positions 12, 13, or 61. Overexpression of the normal protein, usually associated with gene amplification, can have similar effects. The detection of mutationally activated ras genes has been facilitated by the development of oligonucleotide hybridization assays that allow the identification of each possible mutation at the critical sites. Employment of the polymerase chain reaction has greatly increased the sensitivity of these assays. Studies of human lung cancer have shown that adenocarcinoma is the only subtype associated with ras mutations. These occur in about 30% of primary tumors. In almost all cases, the mutation is present in codon 12 of the K-ras gene. No mutations have been observed to date in tumors of nonsmokers, suggesting that the mutation may result from exposure to carcinogenic ingredients of tobacco smoke. Amplifications of ras genes were shown to be very uncommon in clinically early stages of lung cancer. Analysis of the clinical data of patients who were operated on for adenocarcinoma of the lung shows that K-ras mutations are not associated with particular histologic characteristics of the tumors or with specific presenting features. Patients with K-ras mutations, however, had significantly worse survival than did those without an activation.