The factors affecting the occurrence and recurrence of acute otitis media (AOM) were studied among 471 2-3-year-old children in two cities in Finland. Of these children, 188 had experienced greater than or equal to 3 attacks of AOM, 76 had had 1-2 attacks and 207 no otitis attacks (= control group). The study showed that the risk of recurrent AOM was increased among those children attending day-care nurseries as well as among those who had several siblings. Proneness to rhinorrhea and exposure to passive smoking at home was associated with an increased risk of AOM, while prolonged breast-feeding (greater than 6 months) seemed to reduce it. No correlation was found between the risk of recurrent AOM and the place of residence or type of housing, the parental otitis history, or atopic diathesis of a child. Thus the study suggested that to protect a young child from AOM we should promote breast-feeding and home-care for babies as well as avoid smoking in the home.