Light- and electronmicroscopic studies of tissue from pathological middle ears revealed in sixteen cases changes typical of tympanosclerosis, i.e. collagenous connective tissue poor in cells, with hyaline degeneration and occasional calcium deposits. in one case new formed cartilaginous tissue was present. in several specimens we found granulation tissue, which is assumed to represent the incipient stage in the pathogenesis of tympanosclerosis. Electron-microscopy has revealed that the degenerative process involves not only connective tissue, but also the epidermis of the tympanum. It is stressed that the diagnosis of tympanosclerosis can be established only by means of microscopic study of the removed tissue.