Abstract To assess the effect of exercise and to determine the influence of the right ventricular (RV) internal dimension on RV systolic function in morbid obesity, M-mode and 2-dimensional echocardiography and radionudide ventriculography were performed on 22 patients whose body weight was at least twice the ideal body weight and who had no clinical or laboratory evidence of underlying organic heart disease or pulmonary disease. RV ejection fraction was measured at rest and during peak supine bicycle exercise. RV exercise response was defined as the change in RV ejection fraction during peak exercise. There was a significant negative correlation between percent over ideal body weight and RV exercise response (r = 0.86, p < 0.00005) and between RV internal dimension and RV exercise response (r = 0.60, p < 0.005). There were significant positive correlations between resting RV and left ventricular (LV) ejection fraction (r = 0.56, p < 0.01) and between RV and LV exercise response (r = 0.70, p < 0.0005). The subgroup with a high-normal or enlarged RV internal dimension (≥2.0 cm, n = 10) experienced no significant change in RV ejection fraction with exercise, whereas the subgroup whose RV internal dimension was <2.0 (n = 12) experienced a significant increase in RV ejection fraction from 44 ± 10% at rest to 58 ± 11% at peak exercise (p < 0.03). The results suggest that in morbidly obese individuals without underlying cardiopulmonary disease RV dilatation may predispose to RV systolic dysfunction and assessment of RV systolic function should optimally include evaluation of RV exercise response.