BACKGROUND Accumulating evidence suggests that regulatory B cells (Bregs) play an important role in modulating the immune response to tumours. Our previous study indicated that a small percentage of peripheral CD19+CD24hCD27+ Breg cells slowed gastric cancer progression in XELOX-treated patients. Here, we further investigated the relationship between dynamic changes in circulating Breg cells and the clinical course in XELOX-treated gastric cancer patients. METHODS A total of 52 patients with advanced gastric cancer were enrolled in this study. The frequencies of CD19+CD24hCD27+ cells in peripheral blood were tested before (as a baseline) and 9 weeks after administration of oxaliplatin and capecitabine (XELOX). The primary endpoint of the study was progression-free survival time (PFS) of the patients. The overall survival (OS) and adverse events of chemotherapy were also recorded. RESULTS The median PFS of patients was 6 months (95% CI, 5.27-6.73) with effective rate of 46.2%. The percentage of CD19+CD24hCD27+ cells in lymphocytes ranged from 0.007% to 1.94%, with a median value of 0.45%. The median percentage of CD19+CD24hCD27+ lymphocytes was 0.59% (0.01%-6.02%) 9 weeks after treatment. There were no significant differences for this index. However, the patients with decreased Breg frequencies after XELOX treatment had a longer PFS time (7.0 months vs. 5.0 months, p=0.01) than those with increased Breg frequencies. CONCLUSION Patients with downtrend of CD19+CD24hCD27+ B lymphocytes during early stages of chemotherapy relative to their initial values had longer PFS times, and this could be used to predict the efficacy of chemotherapy and help physicians adjust treatments accordingly.