Renal function has been evaluated in 45 diabetic children (age 12.5 +/- 4 years) with a mean diabetes duration of 4.9 +/- 3.5 years. Glomerular filtration rate (GFR; inulin and creatinine clearances), renal plasma flow (RPF; PAH clearance), resting urinary albumin excretion (UAE) were measured and compared with indexes of metabolic control: Hb A1C and blood glucose values (mean, post-prandial and maximal excursion) on the same day. GFR (inulin clearance) and RPF were significantly increased in the diabetic group (171 +/- 31 and 778 +/- 172 ml/min per 1.73 m2) compared with controls (124 +/- 18 and 631 +/- 128 ml/min per 1.73 m2). Both parameters were strongly correlated (r = 0.73; P less than 0.001). Creatinine clearance was not correlated to inulin clearance. Hyperfiltration (inulin clearance above 160 ml/min per 1.73 m2) was noted in 61% of the patients and was independent of diabetes duration. Five diabetic children had a UAE level above 15 micrograms/min. No relationship could be established between UAE and any of the metabolic indexes; GFR was weakly correlated to HbA1C (r = 0.35; P less than 0.05), to mean (r = 0.37; P less than 0.05) and post-prandial blood glucose (r = 0.37; P less than 0.05). In contrast, there was a strong correlation between GFR and the maximal blood excursion (r = 0.62; P less than 0.001). The study shows that renal abnormalities can be detected with a high frequency in diabetic subjects characterized by both an early onset and a short duration of diabetes and suggests the need for a more systematic evaluation of renal parameters in this population.