A retrospective study was conducted on invasive Haemophilus influenzae infections in adults (greater than or equal to 16 years) for the period 1971-1983 in two regions in Sweden. The annual incidence was determined to be 1.1 per 100,000. Predisposing factors included advanced age, bronchopulmonary diseases, alcoholism, traumatic head injury, malignant diseases and pregnancy. Pleuropulmonary infections were the most common manifestations followed by epiglottitis, meningitis and septicaemia of unknown origin. A death rate of 8% was established. Both encapsulated and non-typable strains were found to be potentially pathogenic, but the non-typable strains had a lower virulence.