Haemophilus influenzae type b (Hib) is the leading cause of invasive bacterial disease among children in the United States. Before effective vaccines were introduced, one in 200 children developed invasive Hib disease by the age of 5 years. Sixty percent of these children had meningitis; 3%-6% died. Permanent sequelae, ranging from mild hearing loss to mental retardation, affect 20%-30% of all survivors of meningitis. Ninety-five percent of the cases of invasive H. influenzae disease among children less than 5 years of age are caused by organisms with the type b polysaccharide capsule. Approximately two-thirds of all cases of Hib disease affect infants and children less than 15 months of age, a group for which a vaccine has not previously been available (1).