Background: To investigate dynamic fluctuations of serum viral load and peripheral T-lymphocyte subpopulations of chronic hepatitis B patients and their correlation during entecavir therapy. Methods: Fifty-five patients received entecavir 0.5 mg/d therapy. Serum HBV DNA load was measured by Real-Time-PCR, and the levels of peripheral T-lymphocyte subpopulations by flow cytometry biweekly, every four weeks and every eight weeks during weeks 1-12, 13-24 and 24-48, respectively. Multilevel modelling was used to analyse the relationship between these variables. Results: Of the 55 patients, all HBeAg positive and with detectable HBV DNA, the majority (81.8%) had serum levels of HBV DNA over 107 copies per milliliter. HBV viral load dropped sharply during the first two weeks. In 28 and 43 patients, the level became undetectable from week 24 and 48, respectively. Using pre-therapy level as the reference, a significant decrease in CD8(+) T cells and increase in CD4(+) T cells were found from week 12. Both parameters and CD4(+)/CD8(+) ratio steadily improved throughout the 48 weeks. Multilevel analyses showed that the level of decrement of HBV DNA was associated with the increment of T-lymphocyte activities only in the later period (4-48 week). After 4 weeks of therapy, for each log(10) scale decrement of HBV DNA, the percentage of CD4(+) lymphocyte was increased by 0.49 and that of CD8(+) decreased by 0.51. Conclusion: T-lymphocyte subpopulations could be restored partially by entecavir treatment in patients with chronic hepatitis B concurrently with reduction of viremia.