A total of 938 healthy 3 yr old children, divided into 2 unselected cohorts within the same geographic area, had tympanometric screening and rescreening. The results were assessed according to the parameters, season and child care. In all tests, children in day-care centers had the highest prevalence of secretory otitis and tubal dysfunction due rather to an excess of long courses of illness than of an increased incidence. Regardless of the season, 8-13% of all children in day-care centers exhibited persistent secretory otitis through 6 mo. as compared with 2-4% of children not attending day-care centers. The lowest morbidity rate and the shortest course was found in children cared for at home whose tympanometric results were to a marked extent independent of the season. Children in family day-care had results which in summer corresponded to those in children cared for at home and in winter approximately to those in the day-care centers. The theoretical and practical implications of the findings are discussed.