This study was intended to compare exogenous [13C]glucose (Gluexo) oxidation in boys with insulin-dependent diabetes mellitus (IDDM) and healthy boys of similar age, weight, and maximal O2 uptake. In a control trial with water intake (CT) and in a 13C-enriched glucose trial (GT), subjects cycled for 60 min (58.8 ± 0.9% maximal O2 uptake) while the utilization of total glucose, total fat, and Gluexo was assessed. In CT, total glucose was 84.7 ± 9.2 vs. 91.3 ± 6.6 g/60 min (not significantly different) and total fat was 13.3 ± 2.2 vs. 11.1 ± 1.7 g/60 min (not significantly different) in IDDM vs. healthy boys, respectively. In GT, Gluexo was 10.4 ± 1.7 vs. 14.8 ± 1.1 g/60 min, corresponding to 9.0 ± 1.0 vs. 12.4 ± 0.5% of the total energy supply in IDDM and healthy boys, respectively ( P < 0.05). Endogenous glucose was spared in both groups by 12.6 ± 3.5% ( P < 0.05). Blood glucose and plasma insulin concentrations were two- to threefold higher in IDDM vs. healthy boys in both trials. In conclusion, Gluexo is impaired in exercising boys with IDDM, even when plasma insulin levels are elevated.