We report a rare case of a subdiaphragmatic cyst, diagnosed prenatally during routine ultrasound screening at 25 weeks' gestation. Serial ultrasonographic evaluation demonstrated a slight increase in the size of the cyst during pregnancy. On the basis of the anatomic site and the sonographic features of the lesion, four diagnostic hypotheses were postulated: cystic neuroblastoma, adrenal hemorrhage, duplication of the inferior third of the esophagus and retroperitoneal cystic lymphangioma. A healthy baby, weighing 3300 g and with Apgar scores of 8 and 9, was delivered vaginally at term. He underwent successful surgery at the age of 3 months. The post-operative course was uneventful and the baby is currently doing well. Histology revealed a bronchogenic cyst.