The occurrence of acute otitis media was studied over a one-year period in a total population of 146822 persons living in different parts of Finland. Of these 146822 persons, 4583 experienced a total of 6518 otitis attacks giving an annual incidence rate of 4.44% (4.48% in males and 4.07% in females, P less than 0.001). Acute otitis media was strikingly concentrated in the youngest age groups, with the highest annual incidence rates, in the order of 50%, found in infants younger than 2 years. Half of all the episodes occurred before the age of 33 months. The relative risk of getting acute otitis media was about 200 times higher during the first 2 years of life than as an adult. The occurrence of otitic episodes followed seasonal variations, and in urban areas the incidence was significantly (P less than 0.001) higher than in the countryside.