A 49-year-old woman was admitted to the hospital because of abnormal shadows on a chest roentgenogram that suggested enlargement of bilateral hilar and mediastinal lymph nodes. Chest surgery was performed after a diagnosis of malignant thymoma that was following a left supraclavicular lymph node biopsy. The tumor consisted of several nodules, the largest of which was 6.5 cm. One nodule had invaded the pericardium and left upper lobe of the lung. Metastases to the peribronchial and mediastinal lymph nodes were present along with implantation on multiple pleural sites. Histological examination revealed that the tumor was of a mixed type with features of thymoma and squamous cell carcinoma, and that these two histological types were present in the same nodule with gradual transition between the two. Immunohistochemical surface marker studies revealed that lymphocytes in the squamous cell carcinoma were of the peripheral blood type. We conclude that squamous cell carcinoma may originate in the epithelial cells of a thymoma.