To determine cardiac chamber size, wall thickness and left ventricular (LV) systolic function in morbidly obese patients, M-mode and cross-sectional echocardiography was performed in 62 patients whose body weight was greater than or equal to twice their ideal weight but who were free from underlying organic heart disease and systemic hypertension. The initial clinical protocol consisted of a medical history, physical examination, electrocardiogram at rest, chest x-ray and echocardiogram. Thereafter, each patient underwent gastric restriction. Thirty-four patients returned for follow-up echocardiography 4.3 +/- 0.3 months after substantial weight loss was achieved. For the whole group (n = 62) and LV internal dimension in diastole was enlarged in 24 (39%), the right ventricular internal dimension was enlarged in 20 (32%), the left atrial dimension was enlarged in 25 (40%) and the ventricular septal and LV posterior wall thickness was increased in 35 (56%). In the 34 patients who returned for follow-up, mean body weight decreased significantly, from 135 +/- 8 to 79 +/- 6 kg (73 +/- 4% of the amount over ideal body weight). In the subgroup with low preoperative LV fractional shortening (n = 13), mean LV fractional shortening increased from 22 +/- 2% to 31 +/- 2% (p less than 0.01). This was accompanied by a significant decrease in the mean LV internal dimension in diastole and mean blood pressure. The results indicate that cardiac chamber enlargement, LV hypertrophy and LV systolic dysfunction occur frequently in morbidly obese patients and that LV systolic dysfunction in such persons may improve following substantial weight loss.