The presence and persistence of IgM antibody against hepatitis B core antigen (anti-HBc IgM) and the correlation with other HBV markers were studied in 42 patients, all of whom had acute HBsAg-positive hepatitis but whose subsequent diseases differed. All patients initially had anti-HBc IgM. In 13 out of 15 patients with uncomplicated acute hepatitis, anti-HBc IgM disappeared within 6 months after onset of the disease. In five out of 12 patients, who in spite of transient HBsAg developed chronic liver disease, the anti-HBc IgM persisted for more than 2 years. Among 15 patients with persistent HBsAg, anti-HBc IgM was present from 7 months to more than 8 years. Seroconversion from HBeAg to anti-HBe was observed in seven patients and in five of these anti-HBc IgM disappeared during the follow-up period. These results indicate that anti-HBc IgM can be used as a serological marker of recent or ongoing HBV infection.