T cell receptors (TCR) are a collection of several proteins located in the outer membrane of the T cell. Mature T cell has definitive TCR genes,which may be used as a clonal marker. To investigate whether TCR gene analysis may be useful for diagnosis of myasthenia gravis (MG) with thymoma, we used the cDNA of the β－ subunit of the TCR served as probes to analyze the structure of the TCR β gene and to determine the extent of diversity between MG with thymoma and MG without thymoma. Eighteen patients were clinically diagnosed generalized MG, eight of them had thymoma. Peripheral blood lymphocytes (PBL) were obtained at or shortly before thymectomy (except one, 2 years after thymectomy). When DNA was digested with EcoRI and hybridized with the 32P labeled probes, two germline bands of 10.5kb and 4.0 kb appeared in DNAs from all 18 patients. The 10.5 kb band contains the C β 1 region and the 4.0kb fragment the C β 2 region, respectively. The 4.0 kb band is usually reduced as compared with the 10.5kb band. This can be explained by the preferential rearrangement of the C β 2 gene in T cell with MG. We were able to demonstrate a rearranged band of 9kb in EcoRI－ digested DNAs of four patients. All the four patients had been found to have thymoma , of which 2 were lymphocyte and 2 mixed type. But rearranged bands could not be detected in the DNAs of another four patients with thymoma, of which 2 were epithelial and 2 mixed type. When DNA was digested with HindIII and BglII, germline bands appeared in DNAs from all patients. No rearranged bands could be observed . Single rearranged bands were detected in four patients with thymoma, this showed that specific TCR gene rearrangement may be related to certain types of thymoma . Detection of TCR gene rearrangement might be helpful to the early diagnosis of MG with thymoma . But we were unable to find gene rearrangement in another four thymoma (2 were epithelial and 2 mixed type). There are three possible explanations for the results. First, thymoma has heterogeneity. Second, TCR gene rearrangement only reflects the clonal origin of the T cell tumor. Third, in one patient, peripheral blood lymphocytes were obtained two years after thymectomy, neoplastic T cells maybe disappear. Thus, our results suggested that TCR gene rearrangement in patients with MG may be helpful for diagnosing certain type of thymoma.