A 78-yr-old patient presented with a malignant thymoma with a predominance of epithelial cells. Lymphocyte markers (E rosettes, T-cell-specific xenoantigen and surface Ig) and electrophoretic studies showed that more than 95% of the peripheral blood lymphocytes were T cells. High levels of circulating thymic factor were repeatedly found before treatment (irradiation and chemotherapy). It is suggested that inappropriate hypersecretion of thymic hormone is responsible or can contribute to the immunological abnormalities found in this patient.