Associations of possible risk factors with prevalence of otitis media with effusion (OME) were prospectively studied in a cohort of 250 infants, aged 0-2 years. In order to determine OME, otoscopy and tympanometry were performed at 3-monthly intervals beginning at term date. Eighteen epidemiologically relevant features were inventoried by means of standardized questionnaires. Multivariate analysis controlled for possible confounding factors. Prevalence of OME was most strongly associated with age (P-value < 0.001). Other factors significantly associated with the prevalence of OME (P-value < 0.05) were gestational age, birth weight, breastfeeding, day-care attendance, number of siblings, season, and parent-reported ear infection, hearing loss, mouth breathing and common cold. No significance was found for gender, date of birth, passive smoking, family history of otitis media, parental socio-economic status and histories of snoring and consultation of a physician. In conclusion: both intrinsic and extrinsic factors appear to play an important role in the prevalence of OME. Some of the risk factors appeared to be time-dependent. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.