A 26-year-old man underwent a chest X-ray for an annual check-up, which showed a protrusion of the aortopulmonary window (Fig. 1). Seven months later, he was referred to our hospital without symptoms. He had a past history of epilepsy with medication. He was a never smoker, but he was exposed to organic solvent due to his work as a researcher of carbon fiber. Chest CT revealed a thin-walled multilocular cystic tumor, 9.0 cm in diameter, in the anterior mediastinum. The septum of the tumor was enhanced by contrast medicine (Fig. 2a). The tumor compressed the left brachiocephalic vein, but seemed not to have invaded the surrounding structures. The serum level of carcinoembryonic antigen was slightly elevated (5.4 ng/ml; normal value, less than 5.0 ng/ml); however, other tumor markers including alfa-fetoprotein, beta-human chorionic gonadotropin, anti-acetylcholine receptor antibody, and soluble interleukin-2 receptor were not elevated. Magnetic resonance imaging (MRI) showed a multilocular cystic lesion with focal mural nodules in the anterior mediastinum (Fig. 2b). The preoperative diagnosis was a cystic thymoma, or a malignant lymphoma. We did not perform needle biopsy for diagnosis preoperatively, due to thin cystic wall. We performed a total resection of the tumor through a median sternotomy. The tumor was fully covered with fibrous capsule with partly dense adhesion to the left mediastinal pleura. Needle aspiration cytology of cystic fluid was done during surgery, and the result was class V (suspicion of seminoma) (Fig. 3). The left mediastinal pleura was resected with the tumor. The pathological diagnosis was seminoma with multilocular thymic cysts. Higher magnification showed large polygonal cells with clear cytoplasm, round or oval nuclei, and prominent nucleoli; immunohistochemical stain for c-kit, oct3/4, d2-40, and placental-like alkaline phosphatase stain was positive, and beta-human chorionic gonadotropin stain was negative [1]. So, these cells were diagnosed as seminoma cells. The cysts were lined by only normal squamous epithelium, and cyst walls were characterized by inflammatory changes (Fig. 4a–d). Testicular ultrasound imaging and abdominal, pelvic, and cerebral CTs did not show any apparent tumors or enlarged lymph nodes preoperatively. However, FDG-PET scan revealed an abnormal uptake (maximal standardized uptake value was 6.8) in the right mesenteric lymph node (Fig. 5a). The patient was classified as intermediate risk group due to suspicion of mesenteric lymph node metastasis, so received four courses of BEP (20 mg/m2 cisplatin on days 1–5, 100 mg/m2 etoposide on days 1–5, and 30kU bleomycin on days 1, 8, and 15, repeated 21 days) as adjuvant chemotherapy. Follow-up PET scan revealed no uptake in the right mesenteric lymph node (Fig. 5b). To date, no recurrence or metastasis has been identified for 16 months.