To test the hypothesis that the neuroendocrine (including autonomic) responses to hypoglycemia are dissociated from the symptomatic responses to hypoglycemia in insulin-dependent diabetes mellitus (IDDM) patients with hypoglycemia awareness and during reversal of hypoglycemia unawareness in IDDM, we used the hyperinsulinemic stepped hypoglycemic (5.0, 4.4, 3.9, 3.3, 2.8, and 2.2 mmol/l) clamp technique to quantitate these responses in nondiabetic control subjects and IDDM patients with hypoglycemia awareness and with hypoglycemia unawareness. The latter were restudied after 3 days, 3- 4 weeks, and 3 months of scrupulous avoidance of iatrogenic hypoglycemia. At baseline, symptom responses were virtually nil in unaware patients (P = 0.0001 vs. nondiabetic); these were increased in aware patients (P = 0.0183 vs. nondiabetic). In contrast, several neuroendocrine responses were comparably reduced in both unaware and aware patients: epinephrine (P = 0.0222 and 0.0156), pancreatic polypeptide (P = 0.0004 and 0.0003), glucagon (P = 0.0112 and 0.0109), and cortisol (P = 0.0214 and 0.0450). In initially unaware patients, symptom responses increased (P = 0.0001) during avoidance of hypoglycemia. Demonstrable after 3 days, these were entirely normal after 3-4 weeks and 3 months. In contrast, none of the neuroendocrine responses increased. Thus, we conclude that several neuroendocrine responses to hypoglycemia (including the adrenomedullary and parasympathetic components of the autonomic response) can be dissociated from symptomatic responses in IDDM patients with hypoglycemia awareness and during reversal of hypoglycemia unawareness in IDDM. Avoidance of iatrogenic hypoglycemia sufficient to reverse the clinical syndrome of hypoglycemia unawareness did not reverse the key elements (deficient glucagon and epinephrine responses) of the clinical syndrome of defective glucose counterregulation. This implies that the mechanisms of hypoglycemia unawareness and of defective glucose counterregulation are, at least in part, different in IDDM.