Objectives: The aim of this study was to evaluate the response to a hepatitis B vaccination, and investigate the HBV DNA in subjects with isolated anti-HBc. Methods: 34 subjects with persistent isolated anti-HBc were included in the study. 32 subjects negative for HBsAg, anti-HBs and anti-HBc were included as a control group. They were all vaccinated with Hepaccine at 0, 1 and 2 months, and anti-HBs titers were measured 1 month after the 1st and 3rd vaccinations (1 and 3 months). The HBV-DNA was tested by polymerase chain reaction in subjects with isolated anti-HBc. Results: After the 1st & 3rd vaccinations, the anti-HBs titers$geq$10mIU/ml were 70.6 & 70.6% in isolated anti-HBc group, and 34.4 & 81.2% in the control group, respectively. There were statistically significant differences after the 1st vaccination, but none after the 3rd, between the two groups. In the isolated anti-HBc and control groups, the primary, amnestic and no responses were 0 vs. 46.9%, 55.9 vs. 6.3% and 29.4 vs. 18.8%, respectively. The HBV DNA was not detected in all subjects with isolated anti-HBc. Conclusion: None of the subjects with isolated anti-HBc had a false positive result (primary response); therefore, they should be excluded from vaccination programs in Korea. To differentiate between immunity and occult infections, a single dose of vaccine, with a follow-up anti-HBs test, is preferable for subjects with isolated anti-HBc. An amnestic response indicates late immunity, and no response a suspect occult infection.