Fetal cystic hygroma is a congenital malformation of the lymphatic system appearing as a single or multiloculated fluid-filled cavity, most often in the nuchal region. The malformation is believed to arise from failure of the lymphatic system to communicate with the venous nuchal system. Sometimes the lesion progresses to fetal hydrops, causing fetal death. To further delineate the cause and natural history of this disorder, we have prospectively studied eight cases of cystic hygroma of the neck, detected at gestational ages of 9 to 15 weeks by transvaginal sonography. Three of the eight fetuses survived (37.5%) and were normal at birth. Either hydrops fetalis or intrauterine fetal death occurred in the other five fetuses. In one of these five, therapeutic abortion was induced because of trisomy 21. In another fetus of these five, trisomy 18 was diagnosed by amniocentesis. This pregnancy ended in intrauterine fetal death. The ultrasonic evaluation of the cystic hygromas revealed that those that were reabsorbed in the three ultimately normal viable fetuses were nonseptated cysts, whereas all the four cystic hygromas ending in fetal death or associated with aneuploidy were septated, multilocular hygromas. In another fetus with nonseptated hygroma, nonimmune hydrops developed, which resulted in premature delivery and early neonatal death.