Background: Diabetes is a chronic disease characterized by impaired fasting blood glucose that leads to disturbances in various organs. In this study, we evaluated relationships between tumor size and grade in a population of diabetic and non-diabetic patients with renal cell carcinoma. Materials and Methods: Between 2007-2013, in our clinic radical nephrectomy performed to 310 patients for renal tumors and pathology reported renal cell carcinoma cases were enrolled in the study. Patients with and without a history of diabetes regarding fasting glucose and HgA1c levels were evaluated during surgery for tumor size and Fuhrman grade. Results: Diabetes was found in 95 patients. The mean age of the patients with and without diabetes mellitus was 64.3 (40-79) and 58.4 (31-87) years, respectively. In the diabetes group 51% of patients had a tumor size over 7 cm and 54% a tumor grade over Fuhrman 3. The respective figures in the non-diabetes group were 35% and 30% (p<0.05 in both cases). Conclusions: Renal cancer appears more aggressive in patients with diabetes. In this study lifestyle and risk factors with diabetes regulation were observed to be important for renal cancer patients. Multicenter studies are needed in larger series for more accurate results.