An estimated 25,000 infants are born to hepatitis B surface antigen (HBsAg)-positive women annually in the United States. With no intervention, 40%-90% of these infants will acquire hepatitis B virus (HBV) infection. Approximately 90% of infected infants develop chronic HBV infection, with a 15%-25% risk for premature death from cirrhosis or cancer of the liver. To prevent perinatal HBV transmission, the Advisory Committee on Immunization Practices (ACIP) recommends that infants born to HBsAg-positive women receive postexposure prophylaxis with hepatitis B vaccine (HepB) and hepatitis B immune globulin (HBIG) within 12 hours of birth, and complete the 3-dose HepB series. To determine infant outcomes after postexposure prophylaxis, ACIP recommends postvaccination serologic testing (PVST) at age 9-18 months. To evaluate the implementation of these recommendations, CDC assessed outcomes at age 24 months (through 2011) among infants born to HBsAg-positive women enrolled during 2008-2009 in Enhanced Perinatal Hepatitis B Case Management Projects (EPHBP). Of 4,214 EPHBP-managed infants who completed ≥3 HepB doses, 63.7% had reported PVST results, 13.3% had reported PVST results but infant age was unknown, and 23.0% had no reported PVST results. Of 2,683 infants with PVST results by age 24 months, 93.3% were protected, 1.2% were infected, 3.2% remained susceptible, and 2.3% had indeterminate results. ACIP-recommended postexposure prophylaxis was highly effective among infants who completed vaccination and received PVST. PVST is critical for guiding medical management of infants born to HBsAg-positive women, identifying infants with HBV infection and in need of further care, and monitoring progress toward the elimination of perinatal HBV transmission.