OBJECTIVES: To measure preclinical noninvasive markers of atherosclerosis in youth with type 1 diabetes (T1DM), and to determine their associations between physical activity level and cardiorespiratory fitness (maximal oxygen consumption [VO2max]). STUDY DESIGN: This was a cross-sectional study including 32 patients with T1DM and 42 healthy subjects aged 6 to 17 years. Main outcome measures included arterial flow-mediated dilation (FMD) and intima-media thickness with high-resolution ultrasonography; physical activity by accelerometer (valid 26 patients with T1DM, 35 healthy subjects) and VO2max. RESULTS: Compared with healthy control subjects, patients with T1DM had higher intima-media thickness (mean 0.50 mm [0.48-0.52, 95% CI] vs 0.48 [0.47-0.49], P=.02) and reduced FMD (4.9% [4.1%-5.7%] vs 7.3 [6.4-8.1], P=.001), VO2max (45.5 mL/kg/min [43.0-48.0] vs 48.7 [46.7-50.6], P ≤ .001), total (567.1 [458.6-675.6] vs 694.9 [606.6-883.2] counts per minute, P=.001) and moderate-to-vigorous physical activity. Patients with T1DM who did more than 60 min/day(-1) of moderate-to-vigorous physical activity had similar FMD compared with relatively inactive healthy subjects, but not as high as active control subjects. CONCLUSION: Youth with T1DM present early signs of atherosclerosis, as well as low physical activity level and cardiorespiratory fitness. Endothelial function is enhanced in patients who practice more than 60 min/day(-1) of moderate-to-vigorous physical activity.