Right heart hemodynamic and endomyocardial biopsy abnormalities associated with marked obesity were characterized in 43 obese patients who presented with symptoms of congestive heart failure. Marked obesity was defined as a body mass index greater than or equal to 35 kg/m2. They were compared to a group of 409 patients with similar presentations but normal body mass. Analysis of the 519 patients showed that body mass index was positively correlated with right heart pressures and cardiac output (p less than or equal to 0.0001), pulmonary vascular resistance index (p less than or equal to 0.003) and systolic blood pressure (p less than or equal to 0.0006). Obese patients had elevated right heart pressures, cardiac output (p less than or equal to 0.0001) and pulmonary vascular resistance index (p less than or equal to 0.02) when compared with a group of lean patients with a similar degree of cardiomyopathy. After evaluation, a significantly higher percentage of obese patients were found to have idiopathic dilated cardiomyopathy compared with lean patients. A specific etiology was found in 264 (64.5%) of the 409 lean patients compared with 10 (23.3%) of the obese patients (p less than or equal to 0.0001). The most common finding on endomyocardial biopsy in the obese group was mild myocyte hypertrophy (67%). These data suggest that the cardiomyopathy of obesity exists and may play an important role in a population referred for the evaluation of heart failure.