Introduction. The association of obesity with renal function disorders has been demonstrated by some studies. Objective. The aim of this study was to assess the prevalence of renal insufficiency (RI), measured as glomerular filtration rate (GFR) <60 ml/min and impact of weight loss on kidney function in a cohort of 109 obese patients (body mass index - BMI >25 kg/m2), without previous kidney disease, and who underwent the dietician’s treatment. Methods. According to body mass index (BMI), the patients were classified as overweight (25-29.9 kg/m2, 25 patients), obesity grade I (30-34.9 kg/m2, 37 patients), obesity grade II (35 to 39.9 kg/m2, 23 patients), and obesity grade III (>40 kg/m2, 24 patients). Data on hypertension, cardiovascular diseases, diabetes, family illness history on diabetes, obesity and hypertension, smoking and medications, lipid profile, serum creatinine (sCr) measured before and after diet, and urine examination were collected from the patients’ records. GFR was estimated using MDRD formula (modification of diet in renal disease). Results The patients groups were similar in age and co-morbidities, lipids, and sCr values. In comparison to other obese patients, blood pressure was the highest in obesity grade III patients (p=0.0001). Mean GFR rate before diet was satisfactory in all studied groups. RI was present in 12.8% patients. After diet nine patients still had GFR < 60ml/min, while lipids decreased in all groups. Patients with the highest decrease of BMI also showed best improvement in GFR. The risk for the development of decreased GFR was higher in elderly patients (estimated rate: -0.434, p<0.0001). Conclusion. Obesity is a potentially reversible risk factor for the development of decreased GFR. The relationship between obesity and decreased GFR may be mediated by the presence of known cardiovascular risk factors. In order to clarify the obesity influence on renal functioning, further studies are needed.