Observations are described in 12 massively obese patients (5 women, 7 men), aged 25 to 59 years (mean 37), who weighed 312 to more than 500 pounds (mean 381). Seven patients had had systemic hypertension, 4 hypersomnia or sleep apnea, 2 diabetes mellitus, and 1 patient symptomatic coronary artery disease. Five patients died suddenly from undetermined causes, 2 from right-sided congestive heart failure, 1 patient from acute myocardial infarction; 1 from aortic dissection; 1 from intracerebral hemorrhage; 1 from a drug overdose, and 1 soon after an ileal bypass. The heart weight was increased in all 12 patients. The heart weight to body weight ratio expressed as a percent ranged from 0.22 to 0.61 (mean 0.37) (normal for men 0.42 to 0.46 [mean 0.43], normal for women 0.38 to 0.46 [mean 0.40]). The left ventricular cavity was dilated in 11 patients and the right ventricular cavity in all 12. Only 2 patients (aged 42 and 59 years) had 1 or more major epicardial coronary arteries narrowed greater than 75% in cross-sectional area by atherosclerotic plaque, 1 of whom had no symptoms of myocardial ischemia. Of 664 five-millimeter segments from the 4 major epicardial coronary arteries from 11 patients (mean 60 per patient), 431 (65%) were narrowed 0 to 25% in XSA, 143 (21%) were narrowed 26 to 50%, 73 (11%) were narrowed 51 to 75%, and 17 (3%) were narrowed 76 to 100%.(ABSTRACT TRUNCATED AT 250 WORDS)