KRAS, NRAS or HRAS genes are mutated to encode an active oncogenic protein in a quarter of human cancers. Redox-dependent reactions can also lead to Ras activation in a manner dependent upon the thiol residue of cysteine 118 (C118). Here, to investigate the effect of mutating this residue on tumorigenesis, we introduce a C118S mutation into the endogenous murine Kras allele and expose the resultant mice to the carcinogen urethane, which induces Kras mutation-positive lung tumours. We report that Kras(+/C118S) and Kras(C118S/C118S) mice develop fewer lung tumours. Although the Kras(C118S) allele does not appear to affect tumorigenesis when the remaining Kras allele is conditionally oncogenic, there is a moderate imbalance of oncogenic mutations favouring the native Kras allele in tumours from Kras(+/C118S) mice treated with urethane. We conclude that the Kras(C118S) allele impedes urethane-induced lung tumorigenesis.