To determine the role of insulin clearance in the dawn phenomenon, we studied 10 adolescents with IDDM in comparison to 10 healthy, matched control subjects reported previously. In diabetics, metabolic clearance rate of insulin was calculated during i.v. infusion of insulin from 0100 to 0430 h and from 0430 to 0800 h (0.17 and 0.33 mU/kg/min, respectively), with a Harvard pump, while maintaining nocturnal euglycemia. In controls, metabolic clearance rate of insulin was calculated from the prehepatic insulin secretion rate based on C-peptide levels. In diabetic and control subjects, plasma glucose, free insulin, and glucagon concentrations were similar and did not change during the dawn period. However, metabolic clearance rate of insulin increased during the dawn period in diabetic (9.42 +/- 0.91 to 19.89 +/- 1.52 mL/kg/min, p less than 0.0001) and control subjects (4.87 +/- 1.11 to 9.30 +/- 1.50 mL/kg/min, p = 0.008). Plasma cortisol and adrenocorticotropic hormone levels increased and growth hormone (GH) decreased significantly during the dawn period. Diabetic adolescents had significantly higher plasma GH levels than control subjects throughout the night. We conclude the 1) increased insulin clearance is responsible for the dawn phenomenon in healthy and diabetic adolescents and 2) insulin resistance due to GH is an unlikely cause for the dawn phenomenon because diabetic subjects, despite higher GH levels, maintain plasma glucose levels similar to control subjects without requiring higher plasma free insulin concentrations.