PURPOSE: Many studies have revealed that diabetes mellitus (DM) increases a person's lifetime risk of colorectal cancer and that DM is associated with a worse outcome of colon cancer, but this association is controversial. In this study, we intended to examine the relationship between DM and the long-term outcomes of colorectal cancer. METHODS: A retrospective analysis was conducted on 657 patients who underwent surgery due to colorectal cancer between 1997 and 2004 at Korea Cancer Center Hospital. The operations were done by a single surgeon. With a median follow-up of 4.7 years, we analyzed differences in recurrence-free survival (RFS) and overall survival (OS) between patients with DM and those without DM. RESULTS: Of the 657 patients, 374 (57%) were males and 67 (10%) had DM. There was no difference in age at diagnosis, sex and pathologic stage of colorectal cancer according to the presence of DM. There were no difference in the RFS and the OS of colon cancer between the patients with DM and those without DM. At 5 years, the RFS was 71.3% in diabetic patients vs. 70.4% in non-diabetic patients (P = 0.480), and the OS was 68.8% in diabetic patients vs. 75.0% in non-diabetic patients (P = 0.498). There was no difference in the median survival between the groups (9.6 years in the diabetic group vs. 10.6 years in the non-diabetic group; P = 0.495). CONCLUSION: In this study, we did not find any relation between the presence of DM and either the recurrence or the survival in cases of colorectal cancer. More studies to elucidate whether the influence of DM is directly related to a higher rate of cancer recurrence or survival are needed.