Background & Aims: IL-35, a novel identified inhibitory cytokine specifically produced by regulatory T cells (Tregs), is an Ebi3-IL-12 heterodimer encoded by Epstein-Barr-virus-induced gene 3 (Ebi3) and interleukin-12 alpha (Il12 ). The aim of the study is to determine the expression levels of IL-35 in peripheral blood mononuclear cells (PBMCs) of chronic hepatitis B (CHB) patients in different phases. Methods: A total of 36 treatment naïve CHB patients, including 17 in immune-tolerant phase [group 1, ALT: 21 (12-51) U/L, serum HBV DNA: 2.48 x 10 9 (6.30 x 10 6 -1.20 x 10 10 ) copies/ml] and 19 in immune-clearance phase [group 2, ALT: 221 (71-1530) U/L, serum HBV DNA: 5.10 x 10 8 (1.62 x 10 6 -1.77 x 10 10 ) copies/ml] were enrolled in the study. The relative mRNA expression levels of Ebi3, Il12 and FOXP3 were determined by semi-quantitative PCR. Results: The significant correlations were observed between the expression of Ebi3 and Il12 (r=0.661, P<0.001), Ebi3 and FOXP3 (r=0.388, P<0.05), Il12 and FOXP3 (r=0.431, P<0.01). The relative expression levels of Ebi3 and Il12 in PBMCs were significantly higher in group 2 when compared with those in group 1 (1.46 ± 0.23 vs 0.80 ± 0.10 and 1.26 ± 0.19 vs 0.65 ± 0.09, P<0.05, respectively). Furthermore, the relative expression levels of Ebi3 and Il12 in group 2 were significantly correlated with ALT levels (r=0.473, r=0.474, p< 0.05, respectively), but not with serum HBV DNA levels.