A 33-year old dentist developed fulminant hepatitis. At admission, a test for IgM antibody to hepatitis B core antigen (IgM anti-HBc) was positive, while tests for HBsAg and HBeAg were negative. He was cured of the disease, but in follow-up examinations from 1983 to 1990 IgM anti-HBc was continuously detected with radioimmunoassay while HBsAg and HBV-DNA were absent in the serum. However, HBcAg was found in a biopsied liver specimen and a small quantity of HBV-DNA was detectable by polymerase chain reaction assay. These observation suggest that the continuous detection of IgM anti-HBc without HBsAg in serum is due to persistent HBV infection and HBV replication in the liver.