The incidence of acute epiglottitis in children is declining in the province of Quebec, Canada. In 1988, a PRP-D anti-Haemophilus type B vaccine was introduced into the routine vaccination schedule of 18-month-old children. A substantial reduction in the occurrence of acute epiglottitis was perceived by clinicians. Since 1992, improved new vaccines (PRP-T, HbOC, PRP-OMPC), given to 2-month-old infants, have been expected to increase the efficacy of the immunization program. The impact of the immunization program on preventing acute epiglottitis was verified using a provincial database system called Med-Echo. In the presumably vaccinated target population (0 to 6 years old), 15 children suffered acute epiglottitis in 1993, whereas 97 cases were reported on the average for each yearly period from 1984 to 1987, just before the program's inception. Thus, the incidence of acute epiglottitis in preschoolers was reduced to 15.4% of its former level. The overall efficacy of the immunization program in preventing acute epiglottitis, therefore, is estimated to be 84.6%.