ObjectiveTo determine the incidence and age distribution of invasiveHaemophilus influenzaetype b (Hib) disease in children (0–14 years) in the Sydney Statistical Division (SSD).DesignRetrospective ascertainment from a defined population, 1985–1987. Eligible cases had Hib isolated from a normally sterile site or endoscopically proven epiglottitis.SettingSydney Statistical Division, which had a population of children aged 0–4 years of 229 165 in 1986.PatientsCases were identified from all potential sources of relevant recorded information.Mainoutcomes:There were 292 eligible cases. Among 284 previously well children, those under 18 months of age contributed 81 of 143 cases (57%) of meningitis and 22 of 71 (71 %) of cellulitis/arthritis but only 8 of 91 (11%) of epiglottitis and 4 of 18 (22%) of infection in other foci. Overall, 9% of cases had occurred by 6 months of age, 42% by 18 months and 55% by 24 months. The annual incidence of invasive Hib disease was 38.5 per 100 000 children aged 0–4 years. In areas with the lowest proportion of young children (less than 5.5%), the incidence of Hib disease in the first 12 months of life was significantly lower than in the remainder of Sydney, although there were no differences in overall disease incidence in the age group 0–4 years.ConclusionsThis study indicates that approximately 1 in 500 urban Australian children develop invasive Hib disease by their fifth birthday. Vaccination of children in Sydney with a conjugate Hib vaccine at 18 months of age would result in a greater potential reduction in Hib disease than in the United States, where universal vaccination at this age is current policy.