Data on the prevalence of otitis media with effusion (OME) as shown by serial tympanometry is presented for young children during the first 5 years of life. The children were participants in the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC); a randomly selected 10% (n = 1400) of whom were selected for examination at ages 8, 12, 18, 25, 31, 37, 43, 49 and 61 months. Whilst sex had no effect, there was a decrease in prevalence of OME with increasing age. There was a marked seasonal effect on the prevalence of OME. Bilateral and unilateral OME were significantly more prevalent in the winter than in the summer months (36.6% in February in children aged 8 months compared with 16% at 61 months and 16.4% in August in children aged 8 months compared with 3.1% at 61 months). The results form an important background against which to assess both the results of screening and also the indications for surgical treatment.