We report a severe flare-up in a chronic hepatitis patient due to dual infection with hepatitis B and D viruses during alpha-interferon therapy. Pre-treatment, the patient had detectable levels of both viruses. After 9 months of therapy, an alanine aminotransferase flare with acute hepatic decompensation was detected. Alpha-interferon was discontinued and lamivudine (100 mg once daily) was started, after which the patient reversed slowly. Hepatitis B early antigen (HBeAg) seroconversion with hepatitis B virus-DNA clearance was observed 1 month after the flare; 15 months later, the patient had persistently normal alanine aminotransferase levels with negative results for both serum hepatitis B virus-DNA and hepatitis D virus-RNA. In conclusion, liver disease may be exacerbated during interferon therapy in patients with chronic hepatitis D who are also positive for hepatitis B surface antigen (HBsAg) and HBeAg. Therefore, extra care in monitoring should be considered and strict follow-up is recommended, since clearance of hepatitis D may occur after HBeAg seroconversion in coinfected patients. Lamivudine may be administered early in hepatitis D-RNA/HBsAg-positive patients at high risk of liver failure once a severe flare-up occurs during interferon therapy.