Cardiovascular events remain a leading cause of morbidity and mortality in patients with juvenile-onset, insulin-dependent diabetes mellitus. To examine the extent and severity of the atherosclerotic lesions underlying this excess morbidity and mortality, clinical and angiographic findings were examined in 32 patients with insulin-dependent diabetes and in 31 nondiabetic patients, matched for age and symptoms, undergoing elective cardiac catheterization for evaluation of coronary artery disease. With respect to the individuals without diabetes, patients with insulin-dependent diabetes were significantly more likely to have severe narrowings, to have them in all three major coronary arteries, and to have them in distal segments. Severe narrowing of multiple vessels was significantly more common in men than in women and in individuals with hypercholesterolemia. We conclude that the high risk of cardiovascular events observed in young patients with insulin-dependent diabetes is secondary to advanced atherosclerotic lesions in coronary arteries. Involvement of distal segments of coronary arteries make these patients frequently unsuitable for bypass grafts.