A review of case series of patients with insulin-dependent diabetes mellitus (IDDM) shows an excess frequency of cardiovascular death and probably myocardial infarction. The excess risk is not well quantitated, not clearly associated with poor control of hyperglycemia, and probably not explained by conventional heart disease risk factors. High-density lipoprotein cholesterol levels are not consistently lower in patients with diabetes than in those without diabetes. The relationship of described abnormalities of the coagulation system in diabetes to cardiovascular disease is unknown. Physiologic and pathologic abnormalities are seen in some younger patients without macrovascular disease, but their importance in relation to the excess cardiovascular morbidity and mortality is obscure.