The sera of apparently healthy carriers of hepatitis B virus surface antigen enrolled in the Taiwan Prospective Study were tested retrospectively for anti-HBc IgM, to determine whether this test would be useful in predicting the development of hepatocellular carcinoma (HCC) and/or cirrhosis. In comparison with men who did not develop HCC or cirrhosis, the relative risk of those with anti-HBc IgM elevations was 3.4 and 5.6, respectively. Each of these factors was highly statistically significant, although the difference between them was not. Demonstration of anti-HBc IgM titers of greater than 1:1000 in serum probably reflects ongoing low level viral replication but not recent infection. Anti-HBc IgM appears to be a useful prognostic indicator for the future development of HCC and/or cirrhosis.