A 2-month-old white girl had a congenital, tan-colored, slightly elevated, 3 x 2-cm plaque on the left midback. Clinically, the lesion was suspected to be a solitary mastocytoma. Rubbing the lesion produced a transient erythema and edema that was similar to Darier sign seen in mastocytoma. Analysis of a skin biopsy specimen revealed a smooth muscle hamartoma; special stains did not show evidence of increased numbers of mast cells. This relatively uncommon condition can be confused with a variety of other cutaneous diseases. Light microscopic examination of a skin biopsy specimen establishes the diagnosis. A review of the approximately 50 cases reported in the literature showed that there is no known associated systemic involvement or malignant transformation. The clinical lesions usually become less prominent with time.