The aim of the study is to provide evidence that the lymphocytic infiltration of myasthenia gravis (MG) muscle do not represent a true autoimmune myositis, rather an infiltration by naive lymphocytes derived from lymphocyte-rich thymomas. Muscle biopsies from 179 patients with pure MG, 6 thymoma patients without MG and 15 patients with definite polymyositis were analyzed. In 18 patients with MG (all associated with lymphocyte-rich thymomas) and in two thymoma patients without MG, lymphocytic infiltrates were identified in muscles. By use of immunohistochemistry, we demonstrated that the lymphocytes in MG differ from those in polymyositis, being mature but in contrast to polymyositis naive CD45RA+ T lymphocytes. We suggest that the lymphocytic infiltrates in patients with MG and thymoma represent an infiltration of muscle by thymoma-derived mature but naive T cells. The finding of CD8+CD45RA+ lymphocytes in muscle may signify an underlying thymoma and should not be misdiagnosed as polymyositis.