Four hundred and seventy cases of meningitis caused by Haemophilus influenza in children and 30 cases in adults were identified in Sweden between 1981 and 1983. The age specific incidence in the most susceptible age group (0-4 years) was 31/100,000/year (440 cases), which is higher than previously reported from Europe. A further 30 cases were seen in children aged 5-14. The risk of developing H influenzae meningitis before the age of 15 was 1 in 669. There were 11 deaths (2%) and five cases of serious neurological sequelae among the children. Only 18 children (4%) had predisposing diseases. All but one of the 294 strains of H influenzae from children that had been serotyped were type b. Infections in adults differed from infections in children. Five of the adults died (17%), 12 had important predisposing diseases, and at least six of the infections were caused by non-typable strains. It is concluded that research into the prevention of invasive H influenzae infections in children should have high priority.