Chronic otitis media (COM) is an inflammation that lasts more than 3 months in the middle ear. According to the WHO, COM rate ranges from 1% to 4% depending on the region, Vietnam is 3% to 5%. COM can be dangerous by erosion of the bones which can cause serious complications, surgery indication is absolute, our research refers to 2 diseases: cholesteatoma and grade IV retraction porket (uncontrolled or precholesteatoma). In the past, patients often come to treatment when lession damage and invasive enlargement of the mastoid region even during inflammation stage with serious complications such as meningitis, cerebral abscess . Today dangerous COM is early diagnosis when the lesions are small and discreet; The CT scan of the temporal bone can determined extent of the lesions (focal or spread), mastoid structure. The change of disease and the development in diagnosis are motivation for improvement in treatment. With severe lesions on the sclerosis mastoid, small antre, post-auriculair or antero-auriculair mastoidectomy made a big and safe mastoid cavity which is too large for lesions with many disadvantages, on this case, the close technic mastoidectomy is difficult with high risk of complications and will be dangerous if patients do not return periodic examination and take the second look surgery when suspected recurrent cholesteatoma. Antrotomy transcanal under microscope was reported by Holt J.J in 2008. When compare with post-auriculair and antero-auriculair, the transcanal is the shortest and direct entrance to antre, and well keeping propre mastoid cortex. Although the endoscopy (1990) was used on ear surgery much later than micoscopy (1950), it become the usefull manipulation for endoral and transcanal entrance thanks for small tip and wide fild. Nguyen Tan Phong (2009), Tarabachi M. (2010) reported endoscopy transcanal atticotomy, antrotomy. Nguyen Tan Phong (2010), Tarabachi M. (2013) continue to down the posterior canal wall for the endoscopic transcanal canal wall down (ET CWD) mastoidectomy. This operation is addapted with cholesteatoma or grade IV retraction pocket base on schlerose mastoid and small antre which made a small size of2 mastoid cavity but ensure control of disease and drainage, rapid recovery time, high aesthetics, can improve hearing. To improve the theoretical, indicative, technical contribution to disseminate surgery in ENT specialist we carry out the topic: “Evaluation of results of endoscopic transcanal canal wall down mastoidectomy for dangerous chronic otitis media” with 2 target: 1. Describe the clinical, subclinical characteristics of patients with dangerous chronic otitis media. 2. Evaluation of the results of endoscopic transcanal canal wall down mastoidectomy in patients with chronic otitis media.