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In a Danish school district comprising 387 7-year-old pupils 10 electroacoustic otoadmittance tests showed in the course of one year a 31% total morbidity of middle ear effusion (MEE), while 30% constantly had normal tympanograms on both sides. The point prevalence of MEE ranged from 9 to 3%, highest during the winter and spring months. In most cases MEE was present only at one examination, but in one-quarter of the cases it persisted for 3 months or longer. Spontaneous recovery occurred in a mean of about 2 months. MEE setting in during the period September to February lasted longer than cases setting in at other times of the year. The middle ear status was extremely dynamic, about one-quarter of the children changing status (type of tympanogram) between consecutive tests and 17% 5 or more times. Spontaneous improvement was frequent (90%) and so was complete recovery (76%), but in about one …