To evaluate effective renal plasma flow (ERPF) we performed renal scintigraphies with 99mTc-Mercaptoacetyl-triglycine (MAG3) in nine patients with glycogen storage disease I (GSD I) (age: 16 +/- 7 years). Two patients presented with proteinuria, none showed hyperaminoaciduria, disturbed tubular reabsorption of phosphate or hypertension. 99mTc-MAG3 clearance values were elevated in eight out of nine patients (865 +/- 233 ml/min/1.73 m2 body surface area) and exceeded the age-dependent mean values by 21%-145%. ERPF values in patients with poor metabolic control were higher than in patients with long-term good metabolic control (988 +/- 186 vs. 619 +/- 55 ml/min/1.73 m2; P < 0.05). We conclude that enhanced ERPF is a common finding in GSD I patients, which precedes clinically overt nephropathy. Renal scintigraphy with 99mTc MAG3 is a suitable method for the early detection and monitoring of kidney dysfunction in GSD I.