For six months we observed all 3-year-old children showing type B or C tympanograms at a prevalence study in a geographically limited area (372 ears or 37.2% of the screened total) to study the spontaneous course of middle ear effusions. A considerable test-retest stability was found only for type B. In type C, such stability was equally rare whether the middle ear pressure was highly or only slightly negative. Conversion to type A was observed in about 70% of cases. Ears having a middle ear pressure from -100 to -199 mm H2O almost always returned to normal. The tendency for a negative middle ear pressure or middle ear effusion to develop was the same for both sexes. Conversely, the prognosis of an effusion, once formed, differed significantly, with only girls showing a brief course. Accordingly, a sex-differentiated evaluation appears to be needed in preschool tympanometric screening, and should be included in all clinical considerations when middle ear effusion is demonstrated in young children.