To ascertain risk factors for otitis media with effusion (OME), a cohort of 1439 preschool children, 2 years of age, was investigated by means of tympanometry at 3-monthly intervals until their fourth birthday. Parents were asked about potential risk factors for OME. Data were analysed in a case-control design with incident cases. Age, season, family size, siblings's history of OME, frequent swimming, duration of breast feeding and public day care appear to have a significant effect on OME, even after adjustment for nasal infection. Gender, race, birth weight and passive smoking were not related to OME incidence. With the exception of age and season, the relative risks of environmental factors for OME are always very low. It is concluded that the study of environmental risk factors for OME is necessary to increase the knowledge of the nature of this disease, but that it does not contribute much to medical care at the moment.