Abstract: Objective To investigate the relationship between the severity of alopecia areata （AA）and frequencies of CD4+CD25+Foxp 3 T cells, CD4+ T cells, and CD8+ T cells in peripheral blood of AA patients. Methods Flow cytometry was employed to measure the percentages of CD25+Foxp 3 regulatory T cells and CD25+ T cells among CD4+ T cells, as well as the percentages of CD4+ T cells and CD8+ T cells among T cells from the peripheral blood of 17 severe AA patients, 15 local AA patients and 25 normal controls. Results The percentage of active CD4+CD25+Foxp 3 T cells was significantly lower in the peripheral blood of severe AA patients than that in the normal controls and local AA patients （0.54% ± 0.31% vs 3.21% ± 0.76%, 0.54% ± 0.31% vs 2.71% ± 0.37%, respectively, both P < 0.001）, while no significant difference was found between the latter two groups （P > 0.05）. In severe AA patients, the percentage of CD4+ T cells was significantly lower than that in normal controls （32.61% ± 3.48% vs 43.0% ± 3.63%, P < 0.001）, while the percentage of CD8+ T cells was significantly higher than that in normal controls （40.96% ± 8.54% vs 25.23% ± 2.14%, P < 0.001）. The percentages of CD4+ T cells and CD8+ T cells of local AA patients were 41.25% ± 4.27% and 26.60% ± 2.28%, respectively, without noticeable difference from those of normal controls （P > 0.05）. The percentage of CD8+ T cells of severe AA patients was negatively correlated to that of CD4+CD25+ Foxp 3 regulatory T cells （r = -0.94, P < 0.001）. Conclusion The severity of AA may be related to the decrease of percentage and functionality of CD4+CD25+ T cells in peripheral blood of these patients.