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 marked deformity of the thorax.