Purpose: This retrospective study aimed to determine the effects of diabetes on overall survival (OS) and cancer-specific survival (CSS) in patients with newly diagnosed colon cancers, with particular focus on the impact of diabetes on survival at each stage of colon cancer. Methods: From January 1999 to January 2008, 2762 consecutive patients diagnosed with colon cancer in Taipei Veterans General Hospital were enrolled. The general characteristics as well as presence of diabetes prior to colon cancer diagnosis were identified. Cox proportional hazard analyses were used for prognostic factors determination; and survival was analyzed using the Kaplan-Meier method with log-rank test. Results: A total of 469 patients (17%) had diabetes at diagnosis of colon cancer. Patients with diabetes had baseline characteristics comparable to those without diabetes with the exception that the patients with diabetes were older (>65 years). Diabetes significantly and negatively impacted OS and CSS in multivariate analyses. After adjusting for possible confounding factors, the prognostic impact of diabetes on OS and CSS was particularly significant in patients with stage II colon cancer. Conclusions: Diabetes is a poor prognostic factor in patients with newly diagnosed colon cancer, and it may directly impact the tumor behavior of stage II disease. Further study is required to elucidate the underlying pathophysiologic mechanisms.