In the normal thymus the number of lymphocytes and the proportion of CD4+CD8+ double-positive cells decrease as a function of age. Six thymomas in elderly patients without myasthenia gravis had a large number of double-positive T cells, as seen in the normal thymic cortex. This suggests that the epithelial cells of thymoma have the function of cortical epithelium and can induce precursor T cells to differentiate to the double-positive stage but not further. In seven thymomas with associated myasthenia gravis, however, there was a conspicuous population of mature single-positive T lymphocytes. These mature T cells may include autoreactive T cells.