Trichilemmal (pilar) cyst, a thick walled domed-shaped nodule without a central punctum as seen in epidermoid cyst. Pilar cysts are common, occurring in 5-10% of the population. Greater than 90% occur on the scalp, where pilar cysts are the most common cutaneous cyst. Pilar cysts are the second most frequent type of cyst on the head and neck.Pilar cysts are almost always benign, malignant transformation being extremely rare. Pilar cysts may be sporadic or may be autosomal dominantly inherited. They contain keratin and its breakdown products and are lined by walls resembling the external (outer) root sheath of the hair.In 2% of pilar cysts, single or multiple foci of proliferating cells lead to proliferating tumors, often called proliferating trichilemmal cysts. Proliferating trichilemmal cysts grow rapidly and may also arise de novo. Although biologically benign, they may be locally aggressive, becoming large and ulcerated. Rarely, malignant transformation leads to distant metastases.No absolute clinical criteria can distinguish malignant from benign proliferating pilar tumors. The entire lesion must undergo histological evaluation to assess for malignancy, and atypia and a high mitotic rate are suggestive of malignancy.