Over a period of 18 years 219 consecutive cases of acute epiglottitis were diagnosed and subsequently investigated in order to elucidate the aetiology, epidemiology and outcome of this disease in a well-defined population in Sweden before general vaccination against Haemophilus influenzae type b infection was introduced. Compared with the results from other parts of the industrialized world, high incidence rates were found in both children (14/100,000/year) and adults (2.3/100,000/year). The annual trend showed a significant decline in incidence among children, whereas in adults it remained unchanged. In cases where the aetiological agent could be determined, infection with H. influenzae type b was the main cause of disease in all age groups. However, in adults 27% (6/22) had a disease caused by micro-organisms other than H. influenzae type b that were verified with a blood culture. Sixty-eight per cent had a negative blood culture. The mortality rate was 0.5% (1/219) and 6% (13/219) developed a significant complication of the disease.