We examined the expression of Fas antigen and Bcl-2 protein in thymic tissue surgically resected from 10 patients with myasthenia gravis, using immunocytochemical techniques. Histologically, thymic tissues from 7 myasthenia gravis patients showed hyperplasia, while 3 other patients had thymomas. In hyperplastic thymic tissue, immunoreactivity for Fas antigen was observed mainly in the network of medullary epithelial cells. In contrast, expression of Fas antigen was rare in the cortex. Fas antigen was also detected to some degree in thymoma tissue from 3 patients. Bcl-2 protein was highly expressed in the medullary thymocytes in the hyperplastic thymic tissue, whereas its staining was quite low in myasthenia gravis thymomas. The number of Bcl-2-positive thymocytes in the medulla was significantly greater in the hyperplastic myasthenia gravis thymic tissue than in the control thymic tissue. These findings suggest that Bcl-2 protein may be upregulated in the myasthenia gravis thymus and that this phenomenon may be related to impaired apoptotic cell death of autoreactive thymocytes in myasthenia gravis.