Objective: To determine the presence and correlates of early heart and blood vessel dysfunction in adolescents with type 1 diabetes mellitus (DM) of relatively short duration. Study design: A total of 33 patients with DM (20 male, mean age 15.8 ± 1.3 years, mean DM duration 9.3 ± 3.9 years) and 16 healthy subjects in a nondiabetic control group (7 male, mean age 17.4 ± 1.7 years) underwent (1) ultrasonography of the right carotid artery to assess distensibility, compliance, and intimal-medial thickness (IMT), (2) echocardiographic assessment of systolic and diastolic ventricular function, (3) lipid profile and hemoglobin A_1c, and (4) overnight timed urine collections for albumin excretion rate. Results: Ultrasonography showed significantly lower carotid artery distensibility in the DM group (38.5 ± 8.2 × 10^-3 vs 46.5 ± 11.7 × 10^-3/kPa, P =.01) but no difference in compliance (14.0 ± 3.4 × 10^-7 vs 15.8 ± 2.9 × 10^-7m²/kPa, P =.08) or IMT (0.061 ± 0.013 vs 0.060 ± 0.014 cm, P =.77). Left ventricular (LV) end-diastolic diameter, LV posterior wall thickness, end-systolic wall stress, shortening fraction, ejection fraction, LV mass, and diastolic function were similar in both groups. Total cholesterol, low density lipoprotein, high density lipoprotein, triglycerides, and blood pressure were also similar. The median albumin excretion rate was 4.8 μg/min in the DM group (range 1.1 to 19.2) and 3.0 μg/min in the control group (range 1.4 to 5.8) (P =.03). Hemoglobin A_1c correlated inversely with both distensibility (r = –.43, P =.02) and compliance (r = –.39, P =.032). Conclusions: This study indicates that early changes in macrovascular function, namely lower carotid artery distensibility, may precede abnormalities in cardiac function or in arterial IMT in adolescents with short duration type 1 DM. It also supports a relationship between hyperglycemia and carotid artery dysfunction. (J Pediatr 2000;137:465-9)