Diabetes has been consistently linked to many forms of cancers, such as liver, colorectal, pancreatic, and breast cancer, however, the role of diabetes in outcome among cancer patients remains unclear. In this study, we retrospectively reviewed electronic medical records of 342 inpatients newly diagnosed with NSCLC referred by a teaching hospital cancer center in southern Taiwan between 2005 and 2007 to examine the effects of fasting glucose levels at time of cancer diagnosis on overall survival in patients with non-small cell lung cancer (NSCLC). All patients were followed up until the end of 2010. The Kaplan–Meier method was used to compare survival curves for patients with and without diabetes. The Cox proportional hazards model was used to estimate hazard ratios for the association between diabetes, other prognostic factors and patient survival. We observed that significant prognostic factors for poor overall survival in patients with NSCLC included older age, smoking, poor performance status, advanced stage (stage IIIB or IV), and no cancer-directed surgery treatment. Particularly, we identified that diabetic state defined by fasting blood glucose level ≥126mg/dl was another independent prognostic factor for these patients. Compared with those who had normal range of fasting glucose level (70–99mg/dl), patients with high fasting glucose level (≥126mg/dl) had 69% excess risk of all-cause mortality in patients with NSCLC. In conclusion: Diabetes as indicated by elevated fasting blood glucose was independently associated with a significantly higher risk of all-cause mortality in patients with NSCLC, indicating that diabetes or hyperglycemia effectively controlled may present an opportunity for improving prognosis in NSCLS patients with abnormal glucose level.