A 30-year-old man visited a local hospital with the chief complaints of a cough and tightness of the chest. He was found to have an anterior mediastinal tumor, and was referred to our hospital for treatment. He was diagnosed with a large mediastinal tumor, over 15 cm in long diameter, compressing the heart and superior vena cava. The tumor was resected, and diagnosed as a multilocular thymic cyst. He followed an uneventful postoperative course, and was discharged. Later, he re-visited us with severe cough and bloody sputum. Despite further evaluation, no definitive diagnosis could be made. Chest computed tomography (CT) revealed a mass in the left pulmonary hilum and multiple nodules in the left lung field. The symptoms rapidly worsened until his death. At autopsy, the left lung lesion was diagnosed as hemangiosarcoma. It was also confirmed that the multilocular thymic cyst contained minute germ cell neoplastic (seminomatous) and hemangiosarcomatous components. We speculate that the hemangiosarcomatous component arising in association with the seminoma metastasized to the left lung.