BACKGROUND/AIMS: The function and phenotype of antigen-presenting dendritic cells (DC) have been checked in patients with chronic hepatitis (CH) undergoing vaccine therapy to have insight about the mechanism of this immune therapy. SUBJECTS AND METHODS: Eleven patients with CH due to hepatitis B virus (HBV) were injected with vaccine containing 20 &mgr;g of hepatitis B surface antigen (HBsAg) once at every 2 weeks for 12 consecutive times. The serum interluekin-12 levels were estimated and the function and phenotype of peripheral blood DC were checked before, during and after the vaccine therapy. RESULTS: Sustained normalization of DNA-polymerase activity and alanine aminotransferase were seen in six patients (complete responders [CR]) due to vaccine therapy. The levels of serum interleukin-12 (P<0.01), the stimulatory capacity of peripheral blood DC (P<0.05) and the numbers of CD83-positive mature DC and CD86-positive activated DC (P<0.05) were significantly increased due to vaccination in CH-B patients, especially in younger patients. CONCLUSIONS: A role of DC during vaccine therapy is shown. This study also indicates that more effective vaccine therapy can be developed by upregulating the function of DC in situ in younger patients.