On the basis of intensified surveillance in Finland we report the epidemiology of invasive Haemophilus influenzae type b disease based on 333 consecutive culture-proved cases recorded during 1985 and 1986. The annual incidence rate among children younger than 5 years of age was 52/100,000; 46% of patients had meningitis, 29% had epiglottitis and 25% had other forms of invasive disease. The median age of patients was 27 months, with 45% younger than 2 years of age. Meningitis and epiglottitis were found more often among boys than among girls, whereas the opposite was found among patients with other types of invasive disease (P = 0.015). Among the latter 68% of children with pneumonia or septicemia were 2 years or older compared with 32% of patients with arthritis, cellulitis or pyelonephritis (P = 0.009). These background data are essential for correct interpretation and application of results from trials with H. influenzae type b conjugate vaccines that are currently ongoing in Finland.