The clinical presentation of thoracic mesenchymal hamartomas varies from an asymptomatic chest wall mass to severe respiratory distress resulting from compression of the airways and lungs. We present the findings on fetal US and MRI of a histologically confirmed case. Following surgical resection, pathological examination corresponded to the cross-sectional imaging features with haemorrhagic, cystic and calcified components. An awareness of the characteristic imaging findings will allow accurate diagnosis of this condition, even prenatally, and thus facilitate appropriate perinatal management and surgical planning.