• The reported frequency of invasive Haemophilus influenzae type b disease occurring within 1 year after immunization was compared in American children who received either Praxis Biologics' Haemophilus b polysaccharide vaccine or Connaught Laboratories' Haemophilus b conjugate vaccine during the first year of distribution. All domestic cases reported to the Food and Drug Administration or the Centers for Disease Control were included in the study. An estimated 4.5 million and 2.0 million doses of polysaccharide and conjugate vaccines were administered, respectively. Approximately three cases of early-onset disease (disease developing less than 15 days after vaccination) per million doses were reported for the polysaccharide compared with four cases per million doses for the conjugate vaccine. There were 30.7 reported vaccine failures per million doses of the polysaccharide vaccine compared with 9.0 per million doses of the conjugate vaccine, a 3.4-fold difference. The reporting rate ratios (cases of vaccine failure to cases of early-onset disease) for the polysaccharide and conjugate were 11.5 and 2.3, respectively, a fivefold difference. Thus, compared with recipients of the polysaccharide vaccine, vaccine failures reported among recipients of the conjugate vaccine were 80% fewer than expected.