Kidney function and size were studied in seven well-controlled male Type 1 (insulin-dependent) diabetic patients before and after administration of highly purified human growth hormone for one week. Glomerular filtration rate, renal plasma flow (steady state infusion technique with urinary collections using 125I-iothalamate and 131I-hippuran), kidney size (ultrasonic scanning) and urinary excretion rates of albumin and beta-2-microglobulin were measured. Highly purified growth hormone was injected subcutaneously, 2 IU in the morning and 4 IU in the evening. The growth hormone dosage applied induced an elevation in plasma growth hormone concentration from the normal level seen in these very well controlled diabetics to levels within the range previously demonstrated in normally controlled Type 1 diabetic patients. During the week of growth hormone administration, glycaemic control was maintained unchanged by increasing the insulin dose by 79 +/- 9% (mean +/- SEM). Glomerular filtration rate increased from 122 +/- 3 to 131 +/- 3 ml/min X 1.73 m2 (p less than 0.05) and renal plasma flow increased from 535 +/- 10 to 569 +/- 22 ml/min x 1.73 m2 (p less than 0.05). Kidney size changed from 128 +/- 5 to 133 +/- 5 ml/1.73 m2 (NS). Urinary excretion rates of albumin and beta-2-microglobulin were unchanged. The present findings suggest that the growth hormone elevation typically found in Type 1 diabetic patients with reasonable clinical control, contributes to the enhanced glomerular filtration rate and renal plasma flow present in that disease.