BACKGROUND: It is not known how long children with Haemophilus influenzae serotype b (Hib) vaccine failure retain protective Hib antibody concentrations after infection. The objective of this study was to determine Hib antibody concentrations in children several years after infection and to identify risk factors for low antibody concentrations. METHODS: The families of children from the United Kingdom who developed invasive Hib disease after prior immunization with Hib conjugate vaccine (i.e., Hib vaccine failure) from October 1992 through December 2005 were asked to complete a questionnaire. A blood sample was also obtained from each child. RESULTS: Of 323 families approached, 260 (80.5%) returned a completed questionnaire, and 175 (54.2%) children provided a blood sample. The median age at follow-up was 8.4 years (interquartile range [IQR], 6.2-15.4 years), and the median duration of follow-up was 4.1 years (IQR, 3.5-9.7 years). Twenty-seven children (16.1%) had been born prematurely and/or had an underlying medical condition, and 18 (10.8%) had immunoglobulin deficiency. The median Hib antibody concentration was 0.70 microg/mL (IQR, 0.22-5.8 microg/mL). Overall, 95 children (56.9%) had antibody concentrations