BACKGROUND: We investigated survival outcomes in diabetic patients with non-small cell lung cancer (NSCLC) treated with concurrent metformin and definitive chemoradiation. METHODS: This single-institution, retrospective cohort study included 166 patients with NSCLC who were treated definitively with chemoradiation between 1999 and 2013. Of 40 patients who had type II diabetes, 20 (50%) were on metformin, and 20 (50%) were not on metformin. The primary outcome was overall survival (OS), and secondary outcomes included progression-free survival (PFS), locoregional recurrence-free survival (LRRFS) and distant metastasis-free survival (DMFS). Kaplan Meier method and log-rank test were performed in survival analysis. Cox regression was utilized in univariate analysis of potential confounders. RESULTS: Median follow-up was 17.0 months. Compared with non-diabetic patients, diabetic patients on metformin demonstrated similar OS (16.3 vs. 14.3 mo, P=0.23), PFS (11.6 vs. 9.7 mo, P=0.26), LRRFS (14.1 vs. 11.9 mo, P=0.78), and DMFS (13.4 vs. 10.0 mo, P=0.69). Compared with diabetic patients not on metformin, diabetic patients on metformin also exhibited similar OS (14.3 vs. 19.2 mo, P=0.18), PFS (19.7 vs. 10.1 mo, P=0.38), LRRFS (11.9 vs. 15.5 mo, P=0.69), and DMFS (10.0 vs. 17.4 mo, P=0.12). Identified negative prognostic factors on included squamous cell histology, lower performance status, higher T stage, and non-caucasian ethnicity. CONCLUSIONS: No statistically significant differences in survival or patterns of failure were found among the three cohorts in this small set of patients. No statistically significant differences in survival or patterns of failure were found between the three cohorts in this small set of patients. Though it is possible that metformin use may in fact have no effect on survival in NSCLC patients treated with definitive RT, larger-scale retrospective and prospective studies are implicated for clarification.