Yolk-sac tumor mimics the yolk sac of the embryo, and the presence of alpha fetoprotein in the tumor cells is highly characteristic. We present an 18-year-old boy with primary pulmonary yolk-sac tumor diagnosed postoperatively. A computed tomographic scan revealed a huge intrathoracic soft tissue mass 20 × 25 cm occupying most of the left hemithorax. Two trials of computed tomographic-guided needle biopsy were nonconclusive. A left upper lobectomy was performed with a complete tumor resection. Postoperatively, the patient's alpha fetoprotein (AFP) was 10,512 IU/mL with gradual decline under chemotherapy. The patient is alive 10 months after surgery and is disease free.