Abstract OBJECTIVE: It has been suggested that KRAS and TP53 mutated tumors might influence the phenotypic behavior of left- and right-sided colon tumors. We investigated the incidence of these mutations in left- and right-sided colon tumors and their possible influence on survival in a homogeneous group of patients with Dukes’ C colon cancers. METHODS: The primary tumors of 55 patients with a sporadic Dukes’ C colon cancer, all treated with adjuvant chemotherapy were analyzed for the presence of KRAS and TP53 mutations. Mutation detection of the KRAS and TP53 genes was performed on paraffin-embedded tumor material, using denaturating gradient gel electrophoresis. The 5-yr survival rates of KRAS and TP53 mutated tumors were analyzed regarding right-sided tumors (defined as tumors up to the splenic flexure) and left-sided tumors (defined as tumors from the splenic flexure to the rectosigmoid peritoneal reflection). RESULTS: KRAS mutations occurred more frequently in the right colon compared to the left colon (R = 38% (10/26); L = 10% (3/29); χ 2 test: p = 0.014). KRAS mutations did not influence survival in patients with right-sided colon tumors. Patients with KRAS mutation–negative tumors in the right colon, however, had a significantly worse survival than patients with left-sided KRAS mutation–negative tumors (5-yr survival; R: 34% vs L: 65%, log-rank test: p = 0.007). TP53 mutations of a possible causative nature were found in 24 tumors (44%). Neither the incidence (R = 42% (11/26); L = 45% (13/29)) nor the survival of TP53 mutated tumors differed significantly between left- and right-sided tumors. Furthermore, survival of patients with TP53 mutation–negative tumors did not differ significantly between left- and right-sided tumors. CONCLUSIONS: There seems to be no difference in survival rate between patients with KRAS mutated and KRAS negative Dukes’ C colon tumors; however, KRAS mutations are more frequently found in the right colon compared to the left colon. TP53 mutations do not have predominance for either side of the colon, and there are no differences in survival in patients with left-sided versus right-sided tumors. Patients with KRAS-nonmutated tumors in the right colon did have a worse survival compared to those with such tumors in the left colon. This suggests that other genetic factors may play a role in tumor genesis in this subgroup of patients.