OBJECTIVE: Coronary artery disease (CAD) occurs earlier in life and is more often fatal in people with type 1 diabetes. This excess risk seems to be higher than in those with type 2 diabetes and is poorly explained by conventional risk factors. The role of glycemic control is controversial and has not been previously addressed in a prospective manner using a reliable marker for subclinical CAD, such as coronary artery calcification (CAC), measured by electron beam computed tomography (EBCT). RESEARCH DESIGN AND METHODS: We measured CAC twice during an interval of 2.7 years in 109 men and women with type 1 diabetes (aged 22-50 years). Progression of CAC was found in 21 patients, based on change in the square root-transformed volume score. RESULTS: In multiple logistic regression, CAC progression was associated with baseline hyperglycemia (odds ratio [OR] 7.11, 95% CI 1.38-36.6, P = 0.02), adjusted for the presence of CAC at baseline (P = 0.01), duration of diabetes (P = 0.02), sex (P = 0.09), and age (P = 0.27). There was also a significant interactive effect of higher insulin dose and higher BMI (P = 0.03). CONCLUSIONS: In conclusion, in this young cohort with type 1 diabetes, suboptimal glycemic control (HbA(1c) >7.5%) was a strong risk factor for progression of CAC. Insulin resistance may also play a role.