The authors describe a newborn patient with mesenchymal hamartoma of the chest wall associated with pulmonary hypoplasia. A massive thoracic tumor was diagnosed by prenatal ultrasonography and magnetic resonance imaging at the 28th week of gestation. She was delivered through cesarean delivery at the 36th gestational week. Respiratory distress because of pulmonary hypoplasia necessitated neonatal intensive care. The tumor extensively involved the left hemithorax including all 12 ribs and the first 10 thoracic vertebrae, resulting inmarked deformity of the thorax. At 5 days of age, she underwent the incisional biopsy through a left thoracotomy. Histopathology of biopsy specimens showed multiple components of mesenchymal origin including premature cartilage, bone, and cystic lesions resembling aneurysmal bone cyst. The tumor then showed a rapid overgrowth, but subsequently exhibited a selflimited growth for months, in which her respiratory condition gradually improved to spontaneous breathing without oxygenation support. The present case advocates perinatal preparations for associated pulmonary hypoplasia and conservative management for the neoplasmin fetuses prenatally diagnosed as having this unique pathological entity. The authors describe a newborn patient with mesenchymal hamartoma of the chest wall associated with pulmonary hypoplasia. A massive thoracic tumor was diagnosed by prenatal ultrasonography and magnetic resonance imaging at the 28th week of gestation. She was delivered through cesarean delivery at the 36th gestational week. Respiratory distress because of pulmonary hypoplasia necessitated neonatal intensive care. The tumor extensively involved the left hemithorax including all 12 ribs and the first 10 thoracic vertebrae, resulting in markedly deformity of the thorax. At 5 days of age, she underwent the incisional biopsy through a left Histopathology of biopsy specimens showed multiple components of mesenchymal origin including premature cartilage, bone, and cystic lesions resembling aneurysmal bone cyst. but the following drawings a self over growth for months, in which her respiratory condition gradually improved to spontaneous brea thing without oxygenation support. The present case advocates perinatal preparations for associated pulmonary hypoplasia and conservative management for the neoplasmin fetuses premedically diagnosed as having this unique pathological entity.