Glomerular hyperfiltration is a characteristic functional abnormality in insulin-dependent diabetes mellitus (IDDM) patients, but the underlying mechanisms remain controversial. Supine and ambulant plasma renin activity (PRA) and aldosterone were measured in ten IDDM patients with normal glomerular filtration rate (GFR), in ten IDDM patients with elevated GFR and in ten nondiabetic controls. Basal and stimulated PRA or aldosterone did not differ significantly between the three groups. These results suggest that in insulin-dependent diabetes the glomerular hyperfiltration is not causally related to hyperactivity of the renin-angiotensin-aldosterone system.