The presence of a keratin plug occluding the deep external auditory canal was first noted and documented in the 19th century. It has subsequently been proposed that two different diseases can be responsible for the presence of this type of obstruction within the deep meatus: keratosis obturans and external auditory canal cholesteatoma. Keratosis obturans is characterized by a dense plug of keratin debris located primarily within the deep meatus. There is an associated hyperplasia of the underlying epithelium and evidence of chronic inflammation within the subepithelial tissue. There is no evidence of erosion or necrosis of the underlying bone. In external auditory canal cholesteatoma the significant finding is extensive erosion of the bony external auditory canal by a wide-mouthed sac, lined with stratified squamous keratinizing epithelium, that arises lateral to the tympanic membrane and is located in the inferior portion of the bony external canal. There is frequently evidence of sequestration of the underlying bone.