The stimulus for this transformation is not known but may be trauma, irritation, or inflammation. Few case reports exist of malignant transformation. Exophytic growth with ulceration and foul-smelling discharge may occur. Some lesions may grow very large and cause pressure necrosis on underlying tissues, especially on the scalp. Proliferating pilar tumor has been reported in patients 20-30 years of age.
Patients usually complain of a slowly or rapidly enlarging nodule.
Patients may give a history of recent trauma to the site. The nodule may have been present for months to years without symptoms. The size may range from 1-10 cm, though lesions of up to 25 cm have been reported. Other reported sites include back, chest, axilla, groin, thigh, vulva, and face. The stimulus for changes in these lesions currently is unknown, though trauma, inflammation, and irritation may play a role.