Audiometric recordings of children suffering from chronic mucous effusion of the middle ear have revealed a statistically significant and permanent sensorineural hearing loss of high frequences. Attemps for mechanical or physical factors as explanation are not convincing. We believe that inner ear disturbances in chronic mucous effusion rise by direct intoxication of the basal turns of the cochlea caused by pathologic alterations of mucous membrane tissue. Certain substances like lysozymes or histamine are supposed to diffuse via round window membrane to the inner ear. On the other hand mucous effusion which fills the middle cavity prevents oxygen to diffuse from the middle ear to the inner ear. Maass et al. (1976) and Morgenstern (1977) clearly have shown that under physiological conditions oxygen tension in the perilymph of the basal cochlear turn partly depends on diffusion from the middle ear. The results of our clinical observations indicate that early diagnosis and prompt treatment is necessary to prevent permanent sensorineural hearing loss of high frequencies, especially in infants and children.