A histological and histochemical investigation of the middle ear mucosa and bone structure is presented. 826 specimens of the mucosa and bone were removed at surgery from 102 patients suffering from chronic suppurative otitis media. Changes were found to affect the mucous membrane and the underlying bone both of the middle ear cleft and of the mastoid process, as well as the ossicular chain. The chronic inflammatory process causes in most cases severe and wide-spread lesions of the mucosa and bone, the extent and variety of which must be carefully considered by the surgeon before undertaking the surgical treatment of chronic otitis media. It follows, therefore, that at tympanoplasty operations an adequate curettage has to be carried out both of the tympanic cavity and of the mastoid process, which is usually affected by the inflammatory process. It would, therefore, be more correct to define the disease as chronic otomastoiditis.