• Fifteen children with previous invasive Haemophilus influenzae type b disease were immunized with a Hemophilus-diphtheria toxin mutant protein conjugate vaccine. Serologic responses were compared with those of 31 newly immunized children without previous invasive H influenzae type b disease. Mean levels of antibody to polyribosylribitol phosphate among study children younger than 18 months were 0.086 mg/L before immunization, 0.737 mg/L after first immunization, and 4.453 mg/L after second immunization. In contrast, the comparable mean polyribosylribitol phosphate antibody levels among control children younger than 18 months were 0.107, 3.580, and 63.502 mg/L. A similar pattern of results was found among children aged 18 months or older. Although children with previous invasive H influenzae type b disease do not respond as vigorously to conjugate vaccine as do previously healthy controls, the response is sufficient to justify routine immunization of such children.