The newly available Haemophilus influenzae type b (HIB) protein conjugate vaccines are efficacious among study populations in which a high proportion of infants and children are vaccinated. In this retrospective study, we show the impact of the availability of HIB conjugate vaccines on the incidence of HIB meningitis at Arkansas Children's Hospital (ACH) in Little Rock. The Arkansas State Health Department estimates that only 43% of children in the state younger than 2 years of age have received the appropriate vaccinations. From 1985 through 1987, 27.3 +/- 4 HIB meningitis cases per year were treated at ACH. Although an HIB conjugate vaccine was licensed for 18-month-old children in December 1987, the incidence of HIB meningitis treated at ACH did not decrease significantly; there were 19.0 +/- 2 cases per year from 1988 through 1990. In December 1990, an HIB conjugate vaccine was licensed for use in infants beginning at 2 months of age. From that time through August 1992, there were five cases of HIB meningitis treated at ACH, representing a significant decrease over previous years. Four of these cases occurred in unimmunized infants younger than 6 months of age. The availability of HIB conjugate vaccines for infants has resulted in a dramatic decrease in the number of cases of HIB meningitis treated at ACH, despite a relatively low proportion of infants and children who are receiving vaccination.