Screening tympanometry was performed in January 1976 and in August 1978 on 938 healthy 3-year-old children (1 868 ears), constituting more than 95% of the total population of 3-year-olds living in a Danish provincial municipality at the time of testing. Seasonal influence upon the occurrence of middle ear effusion and tubal dysfunction, not previously quantitated by systematic studies on pre-school children, was thus elucidated by comparing the prevalence of these conditions in winter and in summer. There was a statistically significant preponderance of pathological tympanometry in winter, but the difference was not overwhelming. The findings emphasize that problems relating to a disturbed middle ear function in early childhood are of great practical importance throughout the year.