Haemophilus influenzae type b is a major cause of several infectious syndromes in young children, the most important of which is meningitis. The mortality rate for H. influenzae meningitis in the United States is less than 10%; 10%-15% of survivors are left with neurologic sequelae. Children in day care may be at increased risk for acquiring primary invasive H. influenzae infection, although the mechanism of increased risk and the age of increased susceptibility are not known. Three prospective studies estimated the risk of subsequent or secondary infections as significantly lower than that observed in age- susceptible household contacts; a retrospective analysis suggested a different conclusion. Asymptomatic carriage rates of H. influenzae type b in day care centers vary widely. A retrospective analysis suggested that the administration of rifampin might have prevented subsequent infections in day care contacts; prospective studies are necessary to convincingly demonstrate the efficacy of rifampin prophylaxis among day care contacts. Acquisition of H. influenzae type b carriage has occurred even among those previously treated with rifampin. Until further data are available, rifampin prophylaxis is usually reserved for use in those facilities in which two or more cases of invasive disease have occurred within 60 days. The impact of the H. influenzae type b capsular polysaccharide vaccine on the epidemiology of disease in day care facilities is not known. However, because the immunogenicity and protective efficacy of the H. influenzae capsular polysaccharide vaccine are strikingly age-related, we recommend immunizing children in day care at 24 months.