To evaluate the effects of childhood and poorly controlled insulin-dependent diabetes mellitus (IDDM) on counterregulatory hormone and symptomatic responses to hypoglycemia, we studied 16 nondiabetic children (13 ± 2 yr), 19 nondiabetic adults (26 ± 3 yr), and 13 children with IDDM (14 ± 2 yr, HbA1, 15.1 ± 3.3%) during a gradual reduction in plasma glucose with the glucose-clamp technique. Plasma glucose was reduced from ∼5 to ∼2.8 mM over 240 min with serial assessment of counterregulatory hormone levels and symptom awareness. The plasma glucose level that triggered a sustained rise in plasma epinephrine was consistently higher in nondiabetic children than in adults (3.9 ± 0.06 vs. 3.2 ± 0.06 mM, P < 0.001). Poorly controlled IDDM further elevated the glucose threshold for epinephrine release to normoglycemic levels (4.9 ± 0.2 mM, P < 0.001 vs. both control groups). Age and IDDM also produced an upward shift in the glucose level at which growth hormone release and symptom awareness were initiated. In contrast to the effect on glucose thresholds, maximal epinephrine responses and symptom scores were increased only by age and not IDDM (2-fold higher in children). We conclude that childhood and poor diabetes control independently contribute to an upward shift in glucose thresholds for counterregulatory hormone release and symptom awareness during mild hypoglycemia. Normoglycemic counterregulation may interfere with efforts to control diabetes in young patients.