Children younger than 2 years of age with previous invasive Haemophilus influenzae (Hib) type b disease may not develop protective antibodies to antigens of Hib and may be at risk of developing a second episode of Hib disease. Twenty-three children with prior Hib disease were immunized with Haemophilus b conjugate vaccine (meningococcal protein conjugate). Children 12 to 24 months of age were given one dose of vaccine and children younger than 12 months of age were given 2 doses 2 months apart. Antibody to the polysaccharide capsule of Hib (PRP) was measured by radioimmunoassay. Eighteen children had preimmunization serum antibody concentrations less than 0.150 micrograms/ml. All 18 children responded with greater than 0.150 micrograms/ml of antibody after a single dose of vaccine. Only 1 of the 23 children had a preimmunization serum antibody concentration greater than 1.000 micrograms/ml. Seventeen children ultimately responded with greater than 1.000 micrograms/ml of antibody (P less than 0.0001), concentrations of antibody thought to correlate with protection. Haemophilus b conjugate vaccine (meningococcal protein conjugate) is immunogenic in children with invasive Hib disease. Children younger than 2 years of age with invasive Hib disease should be subsequently immunized with a Hib conjugate vaccine.