Cutaneous blood flow was measured with the laser Doppler technique and by recording cutaneous O2 tension on the forearm and forehead in nine young adult patients with insulin-dependent diabetes mellitus (IDDM) andnine sex- and age-matched healthy control subjects after induction of hypoglycemia. In the healthy subjects, cutaneous blood flow measured with the laser Doppler technique was increased by 120 ± 26% in the forehead (P < 0.01) and 196 ± 50% in the forearm (P < 0.01) at the glucose nadir (blood glucose 1.8 ±0.2 mM) compared with basal blood flow. In contrast, in diabetic patients, cutaneous blood flow was unchanged. The corresponding changes, at the glucose nadir, with cutaneous O2 tension recordings were 286 ± 131% (P < 0.05) in control subjects and −22 ± 15% (NS) in diabetic patients. An impairment of sympathetic nervous function, not detectable by simple cardiovascular tests, could be responsible for the lack of cutaneous hyperemia and sweating and could contribute to unawareness of hypoglycemia in diabetic patients.