Primary liposarcoma of the lung is extremely rare. We report a 28-year-old pregnant woman who complained of dyspnoea during the third trimester. Chest radiography, thoracic ultrasound, CT and MRI showed a huge heterogeneous tumour involving all the left lung and the mediastinum. The tumour was composed of soft tissue, and fatty and cystic components with calcifications. Diagnosis was made on core biopsy under CT guidance. Surgical excision was performed but unfortunately the patient died during the operation.