A case of placental chorioangioma in an infant who experienced transient congestive heart failure is presented. The mechanism for this heart failure was probably due to excessive left to right shunting of blood across the tumor. Electron-microscopic examination revealed the tumor to be composed of endothelial cells and vascular structures of different types. Immunochemistry revealed the lack of normal placental antigens indicating that these tumors are not composed of trophoblastic tissue. Although chorioangiomas occur infrequently, they are the most common benign tumor of the placenta. Most chorioangiomas are small and are discovered only incidentally; however, large tumors may cause severe fetal distress and intrauterine death. This paper illustrates a case in which the chorioangioma caused transient cardiomegaly in the neonatal period. Electron-microscopic and immunochemical studies confirmed the previous suggestion that this tumor is derived fron nontrophoblastic fetal vessels.