W e hypothesize that in patients with insulin-dependent diabetes mellitus (IDDM), recent antecedent iatrogenic hypoglycemia is a major cause of hypoglycemia-associated autonomic failure, a disorder distinct from classical diabetic autonomic neuropathy (CDAN), and that hypoglycemia-associated autonomic failure, by reducing both symptoms of and defense against developing hypoglycemia, results in recurrent iatrogenic hypoglycemia, thus creating a vicious cycle. We used the hyperinsulinemic (12.0 pmol.kg-1.min-1) stepped hypoglycemic clamp technique to assess autonomic and symptomatic responses to hypoglycemia and the insulin infusion test (4.0 pmol.kg-1.min-1) to assess defense against hypoglycemia on mornings before and after clamped afternoon hypoglycemia (approximately 2.8 mmol/liter) and hyperglycemia (approximately 11.1 mmol/liter) in patients with IDDM. Compared with nondiabetic subjects, IDDM with or without CDAN exhibited reduced epinephrine (P = 0.0222 and 0.0040) and pancreatic polypeptide (P = 0.0083 and 0.0056) responses to hypoglycemia. After afternoon hypoglycemia, lower plasma glucose concentrations were required to elicit autonomic and symptomatic responses during morning hypoglycemic clamps in patients without CDAN. At the 2.8 mmol/liter step, mean (+/- SE) epinephrine levels were 1,160 +/- 270 and 2,040 +/- 270 pmol/liter (P = 0.0060), pancreatic and total symptom scores were 22 +/- 3 and 41 +/- 7 (P = 0.0475) after afternoon hypoglycemia and hyperglycemia, respectively. During morning insulin infusion tests after afternoon hypoglycemia, nadir plasma glucose concentrations were 2.6 +/- 0.2 mmol/liter compared with 3.3 +/- 0.3 mmol/liter (P < 0.001) at the corresponding time points after afternoon hyperglycemia. Thus, we conclude: (a) elevated glycemic thresholds for autonomic responses to hypoglycemia are a feature of IDDM per se, not classical diabetic autonomic neuropathy; and (b) a single episode of afternoon hypoglycemia results in both elevated glycemic thresholds for autonomic and symptomatic responses to hypoglycemia and impaired physiological defense against hypoglycemia the next morning in IDDM.