Thymic myoid cells (TMC) are sparse muscle-like cells in the thymic medulla, which are believed to trigger the autoimmune response in myasthenia gravis (MG). Ultrastructural investigations have revealed mature, degenerating, and immature TMC, but the number of TMC in MG patients does not differ from that in controls. We examined the turnover of TMC at the subcellular level, performing an immunocytochemical study with muscle-specific anti-desmin labelling of 10 thymuses derived from MG patients with lymphofollicular hyperplasia and from 8 normal controls. All thymuses examined revealed mature, immature, and degenerating TMC. Mature TMC contained desmin filaments in between Z-discs provided the sarcomeres were arranged in register. Morphological features of degenerating TMC included hypercontracted sarcomeres, cytoplasmic granular debris, chromatin clumping and, occasionally, membrane-bound bodies. Macrophages were not involved in the process. Immature TMC were of small diameter and contained myofilaments not arranged in myofibrils. In an MG thymus, small immature TMC were found clustered with dying TMC. It may be that degeneration of TMC is a stimulus for the generation of new TMC with faster turnover. This mechanism may mean that more antigen is available in MG patients than in normal controls, despite constant TMC numbers.