Objective      To investigate the frequency of intrathymic CD4

+

CD25

+

regulatory T cells (nTreg) and its clinical significance in myasthenia gravis (MG) patients.       Methods      The intrathymic expression of CD25 in 50 MG patients and 20 normal thymus tissues as controls were analysed by immunohistochemical method and the frequency of CD4

+

CD25

+

T cells derived from MG thymus and peripheral blood monocytes were detected in 10 MG patients by flow cytometry.        Results      The expression of CD25 i

n the hyperplastic thymus was much higher (P0.05) than that in the control

and correlated with MGFA classification. The percentage of CD4

+

CD25

+

T cells in MG hyperplastic thymus was higher than that in control (P0.05)

while the percentage in the peripheral blood monocytes was of no significant difference with the control.       Conclusion      The increased frequency of intrathymic CD4

+

CD25

+

regulatory T cells has an important relationship with the destruction of self-tolerance in myasthenia gravis patients. Further investigation on the changes of frequency and function of the intrathymic nTreg cells would conduce to understand the mechanism of MG.