Congenital smooth muscle hamartoma.

Smooth muscle hamartoma is a rare congenital cutaneous hyperplasia of the arrectores pilorum muscles. Clinically it may mimic a solitary mastocytoma or congenital nevocellular nevus. The pattern histopathologically is distinct from all other benign muscle tumors of the skin and is mimicked only by the smooth muscle hyperplasia present in Becker's nevus.