Aim To explore the significance of the change of CD4^＋CD25^＋ regulatory T cells （Treg） in patients with high-risk transient ischemic attack （TIA） in posterior circulation. Methods The frequencies of Treg were detected in 21 patients with high-risk TIA in posterior circulation, 19 patients with dizziness syndrome and 20 health adults by flow cytometer. Results The frequencies of Treg were found to be significantly lower in patients with high-risk TIA in posterior circulation （5.66%±1.91%） than those of dizziness syndrome （9.18%±2.26%） and health adults （9.21%±2.71%）. Conclusions The frequencies of Treg decrease in patients with high-risk TIA in posterior circulation. The decrease of Treg may lead to the breakdown of autoimmune tolerance and participate in the progression of atherosclerosis, which could be one of the pathogenesis of the high-risk TIA in posterior circulation.