The case of a 10-week-old boy with a solitary mastocytoma is reported. The lesion, noticed at birth, was located in an area extending from the dorsum of the left hand to the proximal phalanx of the ring and little fingers. It appeared as a coin-sized, flattened dome-shaped, round tumor with occasional blistering. The web between the two fingers was free of cutaneous changes. The diagnosis was confirmed by the histological characteristics: a densely mastocytic infiltration into the dermis. The present case of solitary mastocytoma, referred to as "mast cell nevus", is unique because of its previously undescribed divided form.