The name proliferating tricholemmal cystic carcinoma communicates the idea that the neoplasm under scrutiny here is malignant, a carcinoma differentiating toward tricholemmal sheath at the isthmus or of a follicle well advanced in catagen. Where it begins is not known for sure, but it is highly likely that most examples originate in a tricholemmal (isthmic-catagen) cyst. In some instances, proliferating tricholemmal cystic carcinoma probably develops de novo from surface and/or infundibular epidermis. Although the term tricholemmal is generic, referring as it does to the entire outer sheath, that is, at the bulb, the stem, and the isthmus, we, unlike Monteagudo,⁷² affirm that for the title of the particular kind of carcinoma under discussion here, "tricholemmal" serves.