Despite the availability of effective hepatitis B vaccines for many years, ove r 370 million people remain persistently infected with hepatitis B virus (HBV).V iral persistence is thought to be related to poor HBV-specific T-cell response s. A phase I clinical trial was performed in chronic HBV carriers to investigate whether HBV DNA vaccination could restore T-cell responsiveness. Ten patients with chronic active hepatitis B nonresponder to approved treatments for HBV infe ction were given 4 intramuscular injections of 1 mg of a DNA vaccine encoding HB V envelope proteins. HBV-specific T-cell responses were assessed by proliferat ion, ELISpot assays, and tetramer staining. Secondary end points included safety and the monitoring of HBV viraemia and serological markers. Proliferative respo nses to hepatitis B surface antigen were detected in two patients after DNA inje ctions. Few HBV-specific interferon γ-secreting T cells were detectable befor e immunization, but the frequency of such responses was significantly increased by 3 DNA injections. Immunization was well tolerated. SerumHBV DNA levels decrea sed in 5 patients after 3 vaccine injections, and complete clearance was observe d in 1 patient. In conclusion, this study provides evidence that HBV DNA vaccina tion is safe and immunologically effective. We demonstrate that DNA vaccination can specifically but transiently activate T-cell responses in some chronic HBV carriers who do not respond to current antiviral therapies. Despite the availability of effective hepatitis B vaccines for many years, ove r 370 million people remain persistently infected with hepatitis B virus (HBV). V iral persistence is thought to be related to poor HBV-specific T-cell response s. A phase I clinical trial was performed in chronic HBV carriers to investigate whether HBV DNA vaccination could restore T-cell responsiveness. Ten patients with chronic active hepatitis B nonresponder to approved treatments for HBV infection were given 4 intramuscular injections of 1 mg of a DNA vaccine encoding HB V envelope proteins. HBV-specific T-cell responses were assessed by proliferat ion, ELISpot assays, and tetramer staining. Secondary end points included safety and the monitoring of HBV viraemia and serological markers. Proliferative respo nses to hepatitis B surface antigen were detected in two patients after DNA inje ctions. Few HBV-specific interferon γ-secreting T cells were detectable befor e immunization, but the frequency of suc h response was significantly increased by 3 DNA injections. Immunization was well tolerated. Serum HBV DNA levels decrea sed in 5 patients after 3 vaccine injections, and complete clearance was observed d in 1 patient. In conclusion, this study provides evidence that HBV DNA vaccination We demonstrate that DNA vaccination can specifically but transiently activate T-cell responses in some chronic HBV carriers who do not respond to current antiviral therapies.