Thirteen fetuses with nonimmune hydrops (22 to 39 weeks of gestation) were evaluated with two-dimensional and M-mode echocardiography. Ten fetuses had cardiovascular abnormalities resulting in heart failure, and three had noncardiac causes of hydrops. In three cases, hydrops was caused by supraventricular tachycardia. One of these fetuses responded to cardioversion at birth, another responded to transplacental digoxin therapy, and the third died with atrial flutter and high-grade atrioventricular block before delivery. There were no cases of "idiopathic" hydrops. Our results show that fetal echocardiography is useful in determining cardiac causes of in utero heart failure resulting in hydrops fetalis. The fetal echocardiogram may also be used in monitoring transplacental therapy of heart failure.