Abstract: Objective：To determine the frequencies of regulatory B cells(Breg) in peripheral blood mononuclear cell(PBMC) in ankylosing spondylitis(AS) patients and its clinical association with AS. Methods：Thirty-eight consecutive patients with AS were included in this study. Based on BASDAI scores, AS patients were divided into active group(n=21) and inactive group(n=17). Thirty age- and sex-matched healthy individuals served as controls. ASDAS-CRP and BASDAI were used as indicators of AS disease activity. Flow cytometry was used to determine the frequencies of CD19+CD24^hiCD38^hiBreg, CD19+CD24^hiCD27+Breg, CD4+CD25^highTreg and CD3+IL-17+CD8-T_H17 cells in the AS patients, and mRNA levels of RORγt and FoxP3 were examined by quantitative real-time PCR. The patients′ perceived disease activity was recorded by BASDAI and ASDAS using serum CRP levels(ASDAS CRP). Results: The percentage of CD19+CD24^hiCD38^hiBreg in AS patients ［(2.10±0.16%)］ was lower than that in healthy control ［(2.10±0.16)% vs(3.44±0.80)%］. Among AS patients, higher proportion of CD19+CD24^hiCD38^hiBreg was observed in active patients compared with inactive patients ［(1.88±0.66)% vs(2.36±0.57)%］. CD19+CD24^hiCD38^hiBreg ratio was significantly and positively correlated with the levels of Foxp3 mRNA(r_{Foxp3 mRNA<.sub>=0.369，P＜0.05), and negatively related to TH17, RORγt mRNA, BASDAI and ASDAS-CRP socre(rTH17=－0.338, P＜0.05; rRORγt mRNA=－0.661, P＜0.01; rBASDAI=－0.411, P＜0.05; rASDAS-CRP=－0.714, P＜0.01), while they were slightly correlated with Treg(rTreg=－0.006, P＞0.05). However, the frequency of CD19+CD24^hiCD27+Breg, another type of defined Breg, did not show significantly different between AS patients and control. but the propotion of CD19+CD24^hiCD27+Breg in active AS patients was lower than that in control or inactive cases, and the differences were statistically significant. In AS group, the frequency of CD19+CD24^hiCD27+Breg was positively correlated with Foxp3 mRNA(rFoxp3 mRNA=0.344, P＜0.05), and negatively correlated to ASDAS CRP(rASDAS-CRP=－0.383, P＜0.05). No significant correlations were observed between the level of CD19+CD24^hiCD27+Breg and TH17, Treg, RORγt mRNA and BASDAI. Conclusion: Our current study revealed that CD19+CD24^hiCD38^hiBreg was the typical abnormal Breg in AS patients. It was significantly decreased in PBMC of AS patients, and correlated with disease activity, suggesting that Breg may play an important role in the pathogenesis of AS. The results of correlation analysis indicated Breg was associated with RORγt mRNA,FoxP3 mRNA and the proportion of TH17 cells in AS.}