A case of an anterior mediastinal cystic hygroma detected prenatally at 22 weeks' gestation is reported. Because of progressive nonimmune hydrops, cardiac compromise, and mediastinal shift compressing the lungs, in utero decompression was successfully performed at 24 weeks. This newborn infant was delivered at 37 weeks' gestation with no respiratory distress. Operative excision of a large thoracic cystic hygroma was performed shortly after birth. At 19 months of age, this patient appeared with unilateral wheezing and fever. Operative findings confirmed recurrence of a cystic hygroma.