Clinical, virological, and histological findings in four HBsAg chronic carriers who cleared HBV markers from both serum and liver following HDV superinfection are described. The patients were long-term HBsAg carriers and all were HBV-DNA/HBeAg positive. Liver biopsy, obtained from three of the patients between 5 and 15 months prior to HDV superinfection, showed chronic persistent hepatitis in two and chronic active hepatitis in one. During the follow-up of 9-19 months, the patients completely recovered from acute delta hepatitis with termination of HBsAg carriage and regression of the histological feature of chronic liver damage. These data demonstrate that sometimes HDV is able to induce a permanent inhibition of its helper virus. HDV superinfection probably enhances the immune clearance of infected cells during the replicative phase of chronic HBV infection.