Thymomas are tumors of thymic epithelial cells. They associate more often than any other human tumors with various autoimmune diseases; myasthenia gravis is the commonest, occurring in 10–50% of thymoma patients, depending on the World Health Organization-defined histologic subtype. Most thymomas generate many polyclonal maturing T lymphocytes but in disorganized microenvironments Failure to induce self-tolerance may be a key factor leading to the export of potentially autoreactive CD4⁺ progeny, thus predisposing to autoimmune diseases. Normally, the master Autoimmune Regulator promotes expression of peripheral tissue-restricted antigens such as insulin by medullary thymic epithelial cells and induction of tolerance to them. The failure of ∼95% of thymomas to express autoimmune regulator is another feature potentially contributing to autoimmunity.