The author emphasized his opinion in the Japanese Congress of oto-rhino-laryngology in 1960 that the mastoidcells should not be excentrated in tympanoplasty for simple chronic otitis media. As for the reason of his opinion he had stated that reaction in the middle ear are different in three different parts, namely the tympanum, the ossicles and the mastoid cells.<br>The epithelium of the mastoid cells is single cuboidal, but in the well developed mastoid it reveals flat cells which resemble endothelial cells. The author could demonstrate phagacytosis of the epithelial cells of the mastoid cells in a case of chronic otitis media. This phenomenon in the mastoid cells epithelium is not so striking, because phagocytosis of the mastoid epithelial cells was found already by Y. Kurita, the author's co-operator, in exprimental otitis media in young pigs.<br>On the other hand, there was able to recognize strong regenerating tendency in the epithelium of the mastoid cells, judging from his view point of pneumatization theory.<br>In clinical cases the histologic findings of the epithelium of the mastoid cells in chronic otitis convinced us that histological changes in the mucosa are a progressive and reparative process, i. e. healing process, of inflammation. The author believes that this healing process of the mastoid cells in chronic otitis migh the accelerated by use of antibiotics.<br>In most cases, otorrhea in acute exacervation of chronic otitis media might be originated from the tympanum or attic and not from the mastoid, because we observed in these cases that the aditus was obstructed by granulation tissue around the ossicles. It means that the mastoid cavity was isolated from the tympanum.<br>Difficulty of epithelization of the mastoid cavity after tympanoplasty has been an annoying problem for clinical otologists. The postoperative epithelization of the mastoid cavity is difficut in the welldeveloped mastoid cells, according to our clinical experience. Even if the epithelization was once completed after the operation several weeks or months later the epithelium usually disappears again and will develope granulation tissue.<br>In the discussion on this problem the author has described mechanism of the process and he pointed out the regeneration and cyst formation of the epithelial cells of the mastoid under the transplanted epidermis. This regeneration of the epithelial cells might cause the atrophy and ablation of the transplanted epidermis.<br>Concerning prevention of this phenomenon he has recommended several methods. Among these the most effective method is to leave the retroauricular wound open after the completed excentration of the mastoid contents.<br>The author would like to emphasize that the mastoid cells should not be excentrated in tympanoplasty in order to avoid this annoying phenomenon.