Objective To evaluate the clinical value of combined detection of T cell receptor rear-rangement excision circles ( TRECs) and CD31+ regulatory T ( Treg) cells for accessing the recent thymic output in patients with chronic hepatitis B. Methods Four groups involving 135 subjects were set up in this study as follows: mild chronic hepatitis B ( Mild CHB, n=35 ) , moderate chronic hepatitis B ( Moderate CHB, n=35 ) , severe chronic hepatitis B ( Severe CHB, n=35 ) and healthy control ( HCs, n=30 ) groups. CD4+CD25+Treg cells in these subjects were sorted out using magnetic cell separation. The ratio of peripheral CD31+Treg cells to Treg cells in each group was analyzed by flow cytometry. Real-time PCR was performed to detect TRECs in CD4+CD25+Treg cells. The percentages of CD3+, CD4+ and CD8+T cell sub-sets were also measured. Results The ratios of CD31+Treg/Treg cells and the numbers of TRECs in pe-ripheral blood of the Moderate CHB and Severe CHB groups were significantly lower than those of the Mild CHB and HCs groups (P<0. 05), while no statistical difference was found between the mild CHB and HC groups (P>0. 05). No significant difference in the percentages of CD3+, CD4+ or CD8+ T cell subsets was observed between the four groups (P>0. 05). CD31+ Treg/Treg cell ratio had a positive correlation with the number of TRECs (r=0. 551, P=0. 014). Conclusions Both CD31+Treg/Treg cell ratio and the number of TRECs were reduced in the peripheral blood of patients with moderate or severe CHB. CD31+Treg/Treg cell ratio and the number of TRECs were positively correlated and could be used as new indices to evaluate recent thymus output.