Published data on the frequency of otitis media with effusion in pre-school children do not give a uniform view about the age-specific rates. A cohort of 1439 2-year-old children in the Netherlands was followed using serial tympanometry on 9 consecutive occasions up to 4 years of age. The screening rate decreased from 87% on the first occasion to 73% on the last round. The prevalence rates in this study are high compared to results quoted in the literature. At least 80% of the children had otitis media with effusion on at least one occasion before the age of four. Distinct seasonal variation was found in the rates. It is postulated that middle ear effusion is a 'physiological' phenomenon at pre-school age with some pathological exceptions.