The aim of this study was to estimate glomerular filtration rate (GFR) in patients with type 2 diabetes mellitus and to search for factors determining GFR. One hundred and eight (70 female, 29 male) patients were included in the study. Serum lipid and HbAlc levels were measured. In random urine samples urinary albumin and creatinine concentrations were determined and albumin / creatinine ratio was calculated. GFR was estimated by using Tc 99m-DTPA with gamma camera. Fundoscopic examination was performed in all patients. A negative correlation between GFR and age (r=-0.25, p<0.03) and a positive correlation between GFR and body mass index were found. There was no correlation between GFR and diabetes duration, blood pressure, serum HbAlc and lipid levels. GFR was significantly decreased in patients with microalbuminuria than normoalbuminuria (p<0.05) and in patients with macroalbuminuria than with microalbuminuria (p<0.005). GFR was also decreased in patients with retinopathy than without retinopathy (p<0.05). There was no significant difference between patients with and without hypertension. GFR was also lower in male patients and in patients using insulin (p<0.05 and p<0.01, respectively). In conclusion, renal function should be evaluated by GFR estimation in type 2 diabetic patients with micro- or macroalbuminuria, with retinopathy and on insulin therapy.