The prevailing blood-glucose level has been found to influence renal haemodynamics in type 1 (insulin-dependent) diabetes mellitus. In a group of 48 type 1 diabetic patients with normal serum creatinine (less than 120 mumol l-1) and without persistent proteinuria, no relationship was present between blood glucose, corrected to near normoglycaemia (6.8 [6.2 to 7.3] mmol l-1 (median [95% confidence interval]), and glomerular filtration rate (GFR), effective renal plasma flow (ERPF) determined with 125I-iothalamate and 131I-hippuran respectively. GFR tended to increase (2 [-1 to +4] ml min-1 1.73 m-2, 0.05 less than P less than 0.10) and ERPF did not change after a blood glucose rise of 7.9 (7.0 to 8.9) mmol l-1, achieved by an intravenous glucose load in 31 patients. The individual changes in GFR and ERPF were correlated (r = 0.60, P less than 0.005). The changes in GFR were inversely related to baseline blood glucose (r = -0.45, P less than 0.02), but not to baseline GFR. GFR increased (3.5 [0 to +12] ml min-1 1.73 m-2, P less than 0.01) if baseline blood glucose was less than or equal to 6.8 mmol l-1 (n = 16) but ERPF did not. Achievement of near normoglycaemia before measurement of kidney function in type 1 diabetes appears to reduce the influence of variation in glycaemia on renal haemodynamics and thus would improve comparison between and within individuals. Moderate hyperglycaemia can cause a small rise in the glomerular filtration rate.