The incidence of congestive heart failure was determined in relation to prior diabetic status in 5,209 men and women aged 30 to 62 years followed up for 18 years in the Framingham study. Men aged 45 to 74 years had more than twice the frequency of congestive failure as their nondiabetic cohorts, and diabetic women had a fivefold increased risk. This excessive risk appears to be caused by factors other than accelerated atherogenesis and coronary heart disease. Even when patients with prior coronary or rheumatic heart disease were excluded, the diabetic subjects had a four- to fivefold increased risk of congestive heart failure. In women (but not men) with prior coronary disease, diabetes also imposed a threefold increased risk of congestive failure. Furthermore, the increased risk of heart failure in the diabetic patients persisted after taking into account age, blood pressure, weight and cholesterol values as well as coronary heart disease. Women with diabetes appeared to be especially vulnerable and, irrespective of coronary disease status, had twice the frequency of congestive heart failure as men. The excessive risk of heart failure among diabetic subjects was confined to those treated with insulin. The data suggest that diabetes is another discrete cause of congestive heart failure and that some form of cardiomyopathy is associated with diabetes, as a result of either small vessel disease or metabolic disorders.