Acquired smooth muscle hamartoma.

Smooth muscle hamartoma is an uncommon, usually congenital, cutaneous hyperplasia of the arrectores pilorum muscles. When it is acquired, it may be confused with Becker's nevus. We report a case of this rare tumor in a 19-year-old man. The disease started several years ago as multiple small skin-colored papules that subsequently coalesced to form a large soft plaque on the back of the left shoulder. The diagnosis of acquired smooth muscle hamartoma was confirmed on histopathology. The patient was reassured about the benign nature of the lesion and was not advised any treatment.