An eight-month-old Japanese boy presented with a brown macule on the right side of the chest. His mother realized it two weeks after his birth, and it swelled several times by rubbing stimulation. A hair growth in the macule was seen one month later, and the swelling did not appear after the hair growth. Physical examination showed an irregular shaped light brown hirsute macule of 7 cm in size. Pathological findings revealed an increase in eosinophilic smooth muscle fibers in the entire dermis unrelated to the hair follicles. Immunohistochemically, the eosinophilic fibers were positive for desmin and actin and negative for vimentin and c-kit. Congenital smooth muscle hamartoma was diagnosed based upon the clinical and histological findings. As spontaneous regression is frequently observed in this disease, we decided to follow up closely without any treatment.
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