Cystic mesenchymal hamartoma is an extremely rare, benign tumor. Rapid growth to a giant size can pose a threat not only in early childhood but also during fetal life. The experience with 2 antenatally diagnosed giant hepatic cysts with widely disparate approaches to management, treatment, and outcome is presented. A giant hepatic cyst was diagnosed on routine screening ultrasound scan. Because of its extremely massive size, the cyst was treated in utero with repeated aspirations, primarily for obstetric considerations. The infant did well, and the lesion was excised laparoscopically during the neonatal period. A second fetus with a giant hepatic cyst was not treated in utero, and the pregnancy continued to term. Nonimmune hydrops fetalis developed, and the fetus was delivered prematurely at 34 weeks. At birth, the infant was noted to have diffuse neurologic injury and no urine output despite normal-appearing kidneys. The lesion was excised during the neonatal period by open laparotomy. Observations at the time of surgery and pathologic studies of the placenta showed aneurysmal dilatation of the placental veins suggesting in utero compression of the fetal intraabdominal umbilical vein. The infant died shortly after birth. The experience with these 2 cases suggests the possibility that giant mesenchymal hamartoma diagnosed in utero may cause umbilical venous obstruction leading to ischemia during fetal life. Decompression of giant hepatic cysts may reverse this phenomenon and allow normal fetal development., (Copyright 2002, Elsevier Science (USA). All rights reserved.)