Trichilemmal or pilar cysts are common intradermal or subcutaneous cysts, occurring in 5-10% of the population.  Greater than 90% occur on the scalp, where trichilemmal cysts are the most common cutaneous cyst. [2, 3] Trichilemmal cysts are almost always benign. They may be sporadic or they may be autosomal dominantly inherited. [4, 5] They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair.
In 2% of trichilemmal cysts, single or multiple foci of proliferating cells lead to tumors called proliferating trichilemmal cysts.  Proliferating trichilemmal cysts are gradually enlarging (up to 25 cm in diameter), exophytic nodules that occasionally ulcerate.  Although biologically benign tumors, they may be locally aggressive. Recurrences and metastases have been observed, with rare malignant transformation. [5, 6]
Erroneously but often called sebaceous cysts, trichilemmal cysts are lined by stratified squamous epithelium similar to that in the isthmus of the hair follicle.  This is the segment between the insertion of the erector pili muscle and the sebaceous gland duct, where no inner root sheath exists. The keratinization is similar to that which occurs in the outer root sheath.  The squamous epithelium undergoes “trichilemmal keratinization” or rapid keratin formation without a granular cell layer, resulting in a cyst wall without a granular cell layer. [2, 7]
Trichilemmal cysts are common, occurring in 5-10% of the population. 
Trichilemmal cysts have no known racial predilection.
Trichilemmal cysts occur more commonly in women than in men. 
Trichilemmal cysts are more common in persons of middle age than in younger persons.