Why adults with the Eisenmenger syndrome fare so much better than other patients with severe pulmonary hypertension is not known, but may be related to unique hemodynamics found only in these patients and in normal fetuses. We used echocardiography to evaluate ventricular morphology and function in 80 subjects: 45 cyanotic adults and 5 cyanotic adolescents with Eisenmenger syndrome, 10 infants with nonrestrictive ventricular septal defect and left-to-right shunt flow (pre-Eisenmenger phase), and 20 fetuses with structurally normal hearts. Cross-sectional morphology of the hearts was the same in all 4 groups with a flat ventricular septum throughout the cardiac cycle and equal thickness of the right and left ventricular free walls (regression slope 0.98, r = 0.97, p <0.0001). This morphology was the same in patients independent of age, defect type, and ventricular function. Right ventricular fractional area change was slightly inferior to that of the left ventricle but normal in most patients with Eisenmenger syndrome (0.47 +/- 0.14 vs 0.51 +/- 0.13, p <0.01). Overall, there was a highly significant linear relation between right and left ventricular function (r = 0.81, p <0.0001). The hearts of patients with Eisenmenger syndrome are more like normal fetal hearts than normal adult hearts. Because of the unique cardiovascular hemodynamics, regression of right ventricular wall thickness does not occur and is likely the reason that patients with Eisenmenger syndrome fare so much better than other adults with severe pulmonary hypertension.