Patients with morbid obesity have high rates of sudden, unexpected cardiac death. The mechanism of death in these patients is uncertain. Twenty-eight patients with morbid obesity (22 sudden cardiac deaths, 6 unnatural deaths) were compared to 11 age-matched nonobese patients with traumatic deaths. Heart weight, left ventricular cavity diameter, left and right ventricular wall thickness, ventricular septal thickness, epicardial fat thickness, and extent of coronary artery atherosclerosis were determined; myocyte size, nuclear size, and degree of interstitial fibrosis were calculated morphometrically. Mean heart weights in the patients with morbid obesity were increased but remained constant as a percentage of body weight. Of the gross parameters, only heart weight and left ventricular cavity size were independent predictors of obesity. Of microscopic parameters, only nuclear area was an independent predictor of obesity. Of 22 patients with morbid obesity, dilated cardiomyopathy was the most frequent cause of sudden cardiac death in (10 patients), followed by severe coronary atherosclerosis (6), concentric left ventricular hypertrophy without left ventricular dilatation (4), pulmonary embolism (1), and hypoplastic coronary arteries (1). The cardiomyopathy of morbid obesity is characterized by cardiomegaly, left ventricular dilatation, and myocyte hypertrophy in the absence of interstitial fibrosis. It is the most common cause of sudden cardiac death in these patients.