AIM:The aim of this study was to seek an association between the control of type 2 diabetes mellitus (T2DM), as determined by hemoglobin A1c (HbA1c) levels, and the outcome of colorectal cancer (CRC). METHODS:We performed a retrospective review of patients with T2DM who had CRC diagnosed between 1997 and 2001. We defined well-controlled T2DM as HbA1c < 7.5% and poorly controlled T2DM as HbA1c > or = 7.5%. A group of age- and gender-matched patients who had CRC without T2DM were used as controls. Forty clinical factors were reviewed, and those associated with poor clinical outcome in each group were examined by univariate analysis (UA) and by the maximum likelihood analysis of logistic regression to determine the independent predictors of cancer outcome. RESULTS:We identified 155 patients with T2DM and CRC, and 114 control patients who had CRC without T2DM. We found no significant differences in any clinical factor by UA between the patients with well-controlled T2DM and the patients who had CRC without T2DM. Compared to both of those patients groups, in contrast, the patients with poorly controlled T2DM had more right-sided CRCs (P = 0.04, OR = 2, 95% CI = 1-4.1), more advanced CRCs (P = 0.02, OR = 2.1, 95% CI = 1-4.4), a younger age of presentation (P = 0.05), greater use of exogenous insulin (P = 0.002), and a poorer 5-year survival (P = 0.001) by UA. Logistic regression showed that poorly controlled T2DM independently predicted the early onset of CRC, a more advanced stage at the time of presentation, poorer 5-year survival, and an increased incidence of right-sided CRCs. CONCLUSIONS:In patients with T2DM who have CRC, poor glycemic control is associated with a clinically aggressive course for the cancer.