- Background Although studies in several cancer types suggest that metformin has antitumor activity, its effect on the outcome of targeted therapies in metastatic renal cell carcinoma (mRCC) is poorly defined. We aimed to analyze the effect of metformin use on the outcome of sunitinib treatment in diabetic patients with mRCC. Patients and Methods We performed a retrospective study of diabetic patients with mRCC, who were treated with sunitinib in 8 centers across 2 countries. Patients were divided into metformin users and nonusers. The effect of metformin use on response rate, progression-free survival (PFS), and overall survival (OS), was tested. Furthermore, univariate and multivariate analyses of the association between clinicopathologic factors and metformin use, and outcome were performed using the entire patient cohort. Results Between 2004 and 2014, 108 diabetic patients with mRCC were treated with sunitinib. There were 52 metformin users (group 1) and 56 nonusers (group 2). The groups were balanced regarding clinicopathologic factors. Clinical benefit (partial response + stable disease) in group 1 versus 2 was 96% versus 84% (P = .054). Median PFS was 15 versus 11.5 months (P = .1). Median OS was 32 versus 21 months (P = .001). In multivariate analyses of the entire patient cohort (n = 108), factors associated with PFS were active smoking and pretreatment neutrophil to lymphocyte ratio > 3. Factors associated with OS were metformin use (hazard ratio, 0.21; P < .0001), Heng risk, active smoking, liver metastases, and pretreatment neutrophil to lymphocyte ratio > 3. Conclusion Metformin might improve the OS of diabetic patients with mRCC who are treated with sunitinib.