Three angiosarcomas of serous membrane are described. One originated in peritoneum, one in pleura, and one in pericardium. The tumor arising in peritoneum was a cystic lymphangiosarcoma and may have been caused by previous therapeutic irradiation for carcinoma of the endometrium. This tumor invaded the peritoneal and right pleural cavities extensively and thus resembled diffuse mesothelioma in its behavior. The tumors arising in pleura and pericardium were hemangiosarcomas.