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To describe changes in the number of cases of Haemophilus influenzae type b (Hib) infections among Canadian children before and after the introductory phases of Hib vaccination.Multicentre case series.All 10 pediatric tertiary care centres across Canada participating in the Immunization Monitoring Program, Active (IMPACT) of the Canadian Paediatric Society and the Laboratory Centre for Disease Control.Children with a Hib infection admitted to any of the participating hospitals from 1985 to 1994. Annual case totals from 1985 to 1990 were determined from records of hospital laboratories or coded discharge diagnoses, or both From 1991 to 1994 intensive case surveillance was conducted on the wards in addition to thorough record searches as above.Estimated annual case totals for 1985-90. For 1991-94 intensive surveillance for quarterly case totals, yearly age distribution of cases, and proportion of recent cases that represent vaccination failures or missed opportunities to prevent infection.The total number of Hib cases from 1985 to 1990 was 2095; from 1991 to 1994, there were 326 laboratory-confirmed cases and 15 probable cases supported by Hib antigen detection. The annual number of cases declined from an estimated 485 in 1985 to 24 in 1994, a decrease of 95.1 percent. The steepest interannual decrease (63.7 percent) occurred between 1992 and 1993, following the introduction of infant-based vaccination programs across Canada. The number of Hib cases involving children most at risk (those 6 to 18 months old) decreased from 78 in 1991 to 4 in 1994. Of the 24 cases in 1994, 6 were categorized as preventable, 1 was fatal, and 8 were vaccine failures (2 of which involved currently used vaccines).The prevalence of Hib infections reported by the IMPACT centres has declined greatly since the introduction of vaccination programs. However, deaths and complications continue to occur, attesting to the need to vaccinate all eligible infants and children against this virulent pathogen.