Proliferating trichilemmal tumor (PTT), also known as proliferating pilar tumor / cyst, is an uncommon benign dermal neoplasm that arises from the outer root sheath of the hair follicle. It is presumed to develop from a pre-existing trichilemmal (pilar) cyst.

PTT is more common among individuals of Northern European descent and is seen most often (approximately 90% of cases) on the scalp, typically in women in their fifth or sixth decade. Isolated cases inherited in an autosomal dominant fashion have been reported.

The malignant transformation of PTT is very rare. Only a small number of malignant proliferating trichilemmal tumor (MPTT) cases with documented invasion and metastasis have been reported. Malignant transformation may be preceded by rapid growth, necrosis, and ulceration.