A regional birth cohort of 5,356 infants was enrolled into a follow-up study in order to investigate the determinants of the development of infectious diseases in children. The infants were examined at three months, six months and twelve months of age by the physicians at the well-baby clinics. At these occasions the number of episodes of acute otitis media in the infants, the duration of breast-feeding, the number of siblings, the type of day care, parental smoking, the presence of animal pets at home and the number and type of other illnesses than otitis were registered. Two thirds of the infants had no episodes of acute otitis media. The median age for the first episode of acute otitis media was eight months. About 10% of the infants had experienced recurrent otitis media (three episodes or more) during their first year of life. Significant associations were found between the occurrence of acute otitis media and the number of siblings, the type of day care, the sex of the infant, the duration of breast feeding, maternal socioeconomic status and prematurity.