A case of mucoepidermold carcinoma in thymus in a 59‐year‐old Japanese female is presented. She died of cardiac tamponade due to tumor invasion after a 5 years clinical course. At autopsy the main tumor was found in the thymic region with metastases to the sternum, regional lymph nodes, pericardial, and left pleural cavity. The mucoepidermold carcinoma might be probably originated from a hen's egg‐sized cyst which was located in the upper posterior aspect of the tumor‐Involved thymus. No teratomatous components were present. The cyst was most likely to be of thymic or bronchogenic cyst origin, though it was not determined, in view of the lining with pseudo‐stratified ciliated columnar epithelium of the cystic wall and the surrounding with the thymic tissue outside. Moreover, there was thymic hyperplasia with germinal center that was compatible with SLE‐like symptoms in her past history and autoimmune nature of the autopsy findings of pulmona
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