The thymus is believed to play a central role in the pathogenesis of Myasthenia gravis (MG). According to a previous hypothesis, MG is initiated within the thymus by immunogenic presentation of locally produced nicotinic acetylcholine receptor (AChR) to potentially autoimmune T cells. Data of 10 consecutive MG patients demonstrate two critical features of MG thymuses that support the concept of intrathymic activation of autoreactive, AChR-specific lymphocytes. Morphologically, the thymuses showed lympho-follicular hyperplasia in nine cases and benign thymoma in one case. The paramount feature revealed by immunohistological double marker analyses was the intimate association of myoid cells (antigen producing) with interdigitating reticulum cells (potentially antigen presenting cells), both of which were surrounded by T3+ lymphocytes in thymus medulla. All 10 thymuses contained T lymphocytes reactive with AChR. This was in contrast to the peripheral immune compartment (blood) where in only 3 of 10 patients, significant T cell responses to AChR were observed. AChR-specific T cell lines could be established from 8 of 10 thymuses, all members of the helper/inducer subset as indicated by the expression of markers T3 and T4.