In search for a normal prototype of the common thick-walled cutaneous cyst (wen), predominantly found on the scalp and usually misnamed "sebaceous cyst," attention was focused on the middle portion of the human anagen hair follicle between bulge and sebaceous gland. It was found that above the zone of sloughing of the inner root sheath, the thus exposed outer root sheath (trichilemma) undergoes a specific type of differentiation in which the cells enlarge vertically, lose their nuclei, and form dense keratin without the intermediate of a granular layer. Similar keratinization takes place in the trichilemmal sac around the catagen hair and produces the telogen club. Trichilemmal keratinization resembles that taking place in wens in structural detail as well as in histologic staining reactions and is dissimilar from epidermis-like keratin formation taking place in the sebaceous duct, the follicular upper portion (infundibulum, pilosebaceous canal) and epidermoid cysts. "Sebaceous cysts" therefore appear to be trichilemmal cysts.