Rhabdomyomatous mesenchymal hamartoma of skin.

A 4-year-old boy was seen at the Kanto Teishin Hospital because of a lesion present at birth. Examination revealed a round, soft, soybean-sized, pedunculated nodule on the midline anterior neck region. Histologically, the most conspicuous aberration was the presence of bundles of striated skeletal muscle fibers throughout the dermis. This nodule was diagnosed as rhabdomyomatous mesenchymal hamartoma of skin, a name first given by Mills. We believe this is the first report of this condition in Japan.