BACKGROUND: An estimated 1 million to 1.25 million people in the United States are chronically infected with hepatitis B virus (HBV) and are at substantially increased risk of developing chronic liver disease, including cirrhosis and primary hepatocellular carcinoma. Immunization with hepatitis B vaccine (HepB) is the most effective means of preventing HBV infection and its consequences. METHODS: To identify and describe children who had not completed the three-dose HepB series, we analyzed data from the 1999 National Immunization Survey (NIS). Among the 2648 children aged 19 to 35 months who did not complete the HepB series, we examined the relationship between the number of doses of HepB received and the number of vaccination visits made, receipt of the birth dose of HepB, age at the time of first vaccination visit (excluding that for the birth dose of HepB), and completion of the 4:3:1:3 series (four doses of diphtheria and tetanus toxoids and pertussis vaccine, three doses of poliovirus vaccine, one dose of measles-containing vaccine, and three doses of Haemophilus influenzae type b vaccine [Hib]). RESULTS: Overall, 11.8% of the children who were included in the 1999 NIS did not complete the HepB series. Among these series-incomplete children, most (79.8%; 95% CI, 77.4%-82.2%) did not receive the birth dose of HepB, and most (80.2%; 95% CI, 77.6%-82.8%) had three or more vaccination visits. Most of the series-incomplete children (87.3%; 95% CI, 85.1%-89.5%) who had three or more vaccination visits received one or two doses of HepB. Among series-incomplete children with at least three vaccination visits, those who did not receive any HepB were more likely to have completed the 4:3:1:3 series (67.1%; 95% CI, 58.8%-75.4%) than those who received at least one dose of HepB (52.7%; 95% CI, 49.0%-56.4%). CONCLUSIONS: Children who did not complete the HepB series fell into three distinct groups: children who made at least three vaccination visits but did not begin the HepB series (n=326); children who made three or more vaccination visits and received one or two doses of HepB (n=1835); and children who made fewer than three vaccination visits (n=487). Different intervention strategies are needed to have an impact on each of these groups, including understanding why parents and providers may not be receptive to HepB, decreasing missed opportunities to administer HepB, and implementing tracking systems such as registries to identify and contact children who are due or overdue for vaccinations.