BACKGROUND Microalbuminuria has been shown to be predictive for clinical diabetic nephropathy. Renal functional reserve (RFR), as a response to protein loading in a short period of time, is a parameter to assess the ability of kidneys to increase the glomerular filtration rate (GFR). The aim of this study was to predict the early phase of diabetic nephropathy by measuring urinary albumin level and RFR capacity in patients with insulin-dependent diabetes mellitus (IDDM). METHODS Twenty-two patients with IDDM were studied: 11 with a disease duration of less than 5 years (group 1) and 11 with a disease duration of more than 5 years (group 2). As the control group, 15 healthy children (group 3) were included in the study. At the beginning of the study, glucose was measured and the urinary albumin/creatinine ratio was calculated. Average glycosylated hemoglobin (HbA1c) over 1 year was determined. After protein loading (red meat containing 2 g/kg of protein), the creatinine clearance was calculated at each hour for a duration of 4 h. The RFR was accepted as the peak percentage increase in GFR over the baseline value. RESULTS Although metabolic control in group 2 was better, the RFR in group 2 was significantly lower than in group 1 (P < 0.05). Urinary microalbumin levels between the groups did not differ (P > 0.05). In two patients in whom microalbuminuria was detected, the RFR was much lower. CONCLUSIONS Detecting lower RFR levels in patients with normal urinary albumin excretion, as well as in patients with microalbuminuria, may support the idea that the RFR capacity is more sensitive than microalbuminuria in assessing the early phase of diabetic nephropathy.