In 373 children aged 9 mo.-to 7 yr, cared for in 7 kindergartens and 1 nursery home, serial tympanometries were carried out at 2 wk intervals. A Madsen Electronics ZS 330 tympanoscope was used for the measurments. To identify A, C1, C2 and B tympanograms, middle ear pressure, compliance and stapedial reflex were measured. After correction for age and other factors which could influence the prevalence of B and C2 tympanograms (number of siblings and parental smoking, which was included in an investigation of the indoor environment of the same children), no significant difference was found in the prevalence of these types of tympanograms, even though the number of children attending the different day-care centers ranged from 19-72. No significant difference was found in the prevalence of The A, C1, C2 and B tympanograms between right and left ears, but there was a significant difference in the prevalence of the A tympanograms in boys and girls, but not in C1, C2 and B tympanograms. The maximum prevalence of B tympanograms was found in 1-yr-old children, decreasing to the age of 7 yr.