Abstract:: A 2‐month‐old white girl had a congenital, tan‐colored, slightly elevated, 3 × 2‐cm plaque on the left midback. Clinically, the lesion was suspected to be a solitary masiocytoma. 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 w
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