Immunohistochemical studies on the difference of the distribution of non-epithelial cells in thymoma tissues either with or without myasthenia gravis (MG) were done using paraffin embedded sections (thymoma with MG: 8 cases, without MG: 14 cases). The formation of the lymph follicles and infiltration of B lymphocytes outside of the lymph follicles within the tumor tissues were seen in 75% and 87.5% of cases with MG respectively. Contrarily, such findings were seen only in 14.2% of cases without MG. S-100β positive lymphocytes were seen in almost all cases more or less, however, significantly increased in number in cases with MG (p<0.01). S-100α positive dendritic cells appearing in the germinal centers in thymoma tissues, most casses were with MG, were significantly increased in number compared with those in inflammatory lymph nodes or tonsils (p<0.01). These findings were thought to be closely related to the manifestation of MG.