Background: DM increases all-cause mortality in the general population. It is also associated with more complications from chemotherapy, increased risk of radiation pneumonitis and worse surgical-wound healing. In this study, we explored the effect of DM on NSCLC overall survival. Methods: This study utilized the surveys and abstracted medical record resources of NSCLC patients studied by CanCORS, an inception cohort of newly diagnosed lung and colon cancer patients from the United States. Patients with NSCLC stage I-IV were included. The log-rank test was used to compare the survival curves of patients with and without DM. Cox proportional hazard models were used to adjust for other variables in the survival model. Results: Of the 2243 NSCLC patients, 359 (16%) had DM. Survival for patients diagnosed with DM was significantly worse than patients without DM [hazard ratio (HR) 1.28; 95% confidence interval (CI) 1.13-1.45]. The median overall survival for patients with and without DM was 561 and 833 days, respectively. DM was also associated with poorer survival (HR 1.20; 95% CI 1.05, 1.36) after adjusting for age, smoking status, stage, treatment (radiation, chemotherapy), brain metastasis, severity of respiratory symptoms at diagnosis, and comorbidities. While a significant interaction between stage and DM was not detectable after adjusting for the above, the estimated Kaplan-Meier plots present more prominent differences in stages I-III, with virtually overlapping plots in stage IV. Conclusions: DM was associated with worse survival in NSCLC patients. Although this finding can be utilized when counseling diabetic NSCLC patients, it is unknown if improving DM care will result in a better survival. The effect of DM on survival in the various stages of NSCLC needs further study.