Extrathoracic metastatic malignant thymoma. Diagnosis by aspiration cytology. A fine-needle aspirate from a hepatic mass in a 65-year-old white man was examined. The patient had a history of histologically confirmed thymoma with metastases in multiple thoracic sites. The smears showed a dimorphic population of epithelial cells and lymphocytes, characteristic of thymoma. The resemblance of the cytologic pattern to the primary neoplasm and positive reactivity with various immunohistochemical agents peculiar to thymomas confirmed the diagnosis. Aspiration cytology has been described in the diagnosis of primary thymoma. To our knowledge, this is the second reported case of a cytologic diagnosis of metastatic thymoma and the first one with immunohistochemical studies. 