Lymphocyte subsets of the peripheral blood in 43 patients with myasthenia gravis (MG) were determined by two-color flow cytometry using a number of monoclonal antibodies.In the MG patients without thymectomy (Tx) and prednisolone (PSL) treatment, lymphocyte counts, B-cells, CD4+cells and their subsets were normal, but numbers of T-cells, CD8+cells and CD8+CD 11−subsets were significantly decreased. Furthermore, proportions of activated cells in T-cells, CD 16+Leu7–and CD16+Leu7+NK subsets were significantly high in the patients. The changes in T-cells, CD8+cells and activated T-cells were less marked in the MG patients than Sjögren's syndrome (SS) used as a disease control. Contrary to MG patients, lymphocyte counts, CD4+cells and their subsets were decreased, and the proportions of B-cells were high in SS patients. These results suggest altered immunologic conditions, immunologically active and deficient conditions, in both diseases, although the alterations were more prominent in SS than MG.PSL treatments and Tx significantly altered the lymphocyte profiles: PSL decreased lymphocytes, B-cells, T-cells, CD4+cells and their subsets, while the proportions of CD8+cells were increased. The changes were compatible with the known immunosuppressive effects of PSL. After Tx, lymphocytes and B-cells decreased, but the proportions of T-cells, CD8+cells and their subsets, and NK cells subsets returned toward normal. CD4+CD8+cells were not increased in MG patients. and the cells did not decrease after Tx. Some of these observations might be relevant to clinical effects of Tx, although the mechanism responsible for these changes is still unknown.