Congenital smooth muscle hamartoma: the importance of differentiation from melanocytic nevi.

Congenital smooth muscle hamartoma (CSMH) is a rare cutaneous lesion commonly seen on the trunk or extremities as a localized skin-colored or mildly hyperpigmented and irregularly shaped patch or plaque with prominent vellus hairs. Histologically, CSMH is characterized by the proliferation of intersecting bundles of smooth muscle. The clinical differential diagnosis of CSMH includes congenital melanocytic (pigmented) nevus, Becker's melanosis, solitary mastocytoma, piloleiomyoma, café-au-lait spots, and nevus pilosus. Differentiating CSMH from a congenital melanocytic nevus avoids the unnecessary excision of this benign condition.