Hepatitis B surface antibody titers were routinely measured in 150 children with sickle cell disease (SCD) after immunization, and the seroconversion rate was found to be lower than that in the general population (89% vs. 97%, P = 0.002). Most of the children who did not seroconvert after the series of 3 immunizations responded to booster injections (93%). Therefore, we recommend the measurement of hepatitis B surface antibody titers after immunization in those children with SCD at greatest risk for hepatitis B infection. An additional dose of hepatitis B vaccine should be administered to those without evidence of seroconversion.