To assess cardiac morphology and left ventricular (LV) function in normotensive morbidly obese patients with and without congestive heart failure (CHF) we performed a physical examination and obtained a transthoracic echocardiogram and cardiac Doppler studies before and after substantial weight loss in patients whose actual body weight was initially equal to or more than twice their ideal body weight and who were free from systemic hypertension and underlying organic heart disease. There were 24 patients with CHF, 14 of whom were studied after weight loss. There were 50 patients without CHF, 39 of whom were studied after weight loss. Compared to patients without CHF, those with CHF had significantly greater mean LV internal dimension in diastole, LV end-systolic wall stress, LV mass/height index values, left atrial dimension and right ventricular internal dimension values, significantly lower mean LV fractional shortening, and transmitral Doppler E/A ratio values, and significantly longer mean transmitral E-wave deceleration time and duration of morbid obesity than patients without CHF. Substantial weight loss in those with and without CHF produced comparable reductions in mean LV internal dimension in diastole, LV end-systolic wall stress, LV mass/height index, transmitral Doppler E-wave deceleration time, and left atrial dimension, and comparable increases in LV fractional shortening and transmitral Doppler E/A ratio. Linear regression analysis identified duration of morbid obesity as the strongest predictor of CHF (p <0.00000002). Thus, LV mass is greater and LV systolic function and diastolic filling are more impaired in normotensive morbidly obese subjects with CHF than in those without CHF. Duration of morbid obesity is the strongest predictor of CHF among the variables studied. Substantial weight loss produces comparable changes in cardiac morphology and function in those with and without CHF.