In a retrospective study of 179 otitis-prone children and 305 controls, various possible predisposing factors for acute otitis media (AOM) were compared. The children were matched with the controls for age and sex. There were 61% boys and 39% girls in the otitis-prone group and 58% boys and 42% girls among the controls. Information about the family and living conditions, the children's illnesses, ear, nose and throat (ENT) operations and possible allergies were obtained from a questionnaire, and the children were called for a physical examination. The otitis-prone children had more middle-ear problems with pathological tympanograms and conductive hearing loss than the controls. No differences were found in bacterial colonization of the nasopharynx. Besides AOM and secretory otitis media, the otitis-prone children had more other ENT diseases and had consequently undergone more ENT operations and hospitalizations than the controls. There were no differences between the two groups regarding allergy, day care or parental smoking alone, but on comparing children with combinations of these factors there were more otitis-prone children than controls exposed, indicating an additive effect. The combination of different factors, less important separately, may for some children mean the difference between becoming otitis-prone or not.