OBJECTIVE--Increased physical activity and physical fitness are recommended therapeutic modalities in addition to insulin and diet in the management of children with IDDM. The aim of this study was to assess the fitness levels of adolescents with IDDM compared with healthy control subjects and to evaluate the relationship between physical fitness and metabolic control. RESEARCH DESIGN AND METHODS--We studied 59 patients with IDDM, 28 boys and 31 girls, age 15.6 +/- 2.5 yr, duration of diabetes 7.6 +/- 3.5 yr, HbA1 10.6 +/- 2.1% (mean +/- SD), and compared them with 18 healthy, nondiabetic control subjects, 9 boys and 9 girls, matched for age, BMI, and Tanner stage. Physical fitness was measured by VO2max during progressive bicycle ergometry. HbA1 was used to determine glycemic control. Lipid profile included fasting total cholesterol, HDL, LDL, Lp(a), and TG levels. RESULTS--Patients with IDDM had lower VO2max levels than control subjects (33.7 +/- 7.0 vs. 41.0 +/- 10.4 ml.kg-1.min-1, P = 0.001). Males with IDDM had lower VO2max than male control subjects, but diabetic and control females showed no difference. In IDDM patients, VO2max correlated inversely with HbA1, insulin dose, cholesterol, LDL, TGs, and Lp(a), but did not correlate with HDL, which correlated inversely with BMI. CONCLUSIONS--We conclude that the state of physical fitness is an important correlate of lipid levels and Lp(a) in adolescents with IDDM. We speculate that higher physical fitness levels in adolescents with IDDM may decrease the risk of CVD through modulating lipid levels.