Type 1 diabetic patients have a pronounced risk of premature coronary artery disease and death. We sought to evaluate the prevalence of silent coronary atheromatosis and to evaluate the relation between coronary atheromatosis and glycemic control. Coronary atheromatosis was evaluated in type 1 diabetic patients with no symptoms of coronary artery disease by exercise electrocardiogram (ECG) in 39 patients and quantitative coronary angiography and by intravascular ultrasound (IVUS) examinations in 29 patients. The findings from the IVUS examinations were related to mean HbA_1c collected prospectively over 18 years. Abnormal exercise ECGs were found in 15% of patients, and angiographic diameter stenosis of >50% in one or more of the main coronary arteries was found in 34% of patients. All patients examined with intracoronary ultrasound had developed atherosclerotic plaques with an increased intimal thickness (>0.5 mm) in one or more of the coronary arteries. Coronary artery plaque formation, as judged by ultrasound, was significantly related to mean HbA_1c during 18 years (P < 0.05) after adjustment for total cholesterol and age. This study demonstrates a high prevalence of silent coronary atheromatosis in type 1 diabetic patients with no symptoms of coronary heart disease. Long-term glycemic control was shown to be associated with coronary atheromatosis.