To investigate plasma glucagon counterregulatory responses to hypoglycemia, an intravenous insulin bolus was given over 2 min to 73 children, aged 8.5-18.8 yr, with diabetes duration 1.2-17.1 yr. The plasma glucagon responses of the 61 children without glucagon antibodies or abnormal glucagon molecules were compared with those of 13 nondiabetic control subjects, aged 8.3-18.3 yr. Glucagon increments from baseline (73 +/- 10 pg/ml) and peak glucagon responses (212 +/- 13 pg/ml) were markedly lower in diabetic patients than in control subjects (341 +/- 49 and 462 +/- 51 pg/ml, respectively, P less than .001). Glucagon responses were found to correlate positively with the age of the patients at the time of testing (r = .478, P less than .001) and inversely with metabolic control as measured by glycosylated hemoglobin (r = -.342, P less than .02). There was no relationship between glucagon responses and diabetes duration. There was also no relationship between the glucagon increments and free-insulin levels during the test. Glucose recovery from the nadir was impaired in diabetic subjects compared with control subjects and correlated inversely with free-insulin levels. However, glucose recovery did not correlate with the rise of plasma glucagon. Glucose recovery was not different in patients with glucagon antibodies. In this study, we have demonstrated a deficient glucagon response to hypoglycemia in children with insulin-dependent diabetes mellitus. However, the clinical significance of this deficit is not clear.