A case of mastocytosis of the bone associated with urticaria pigmentosa was first suspected of being a generalized metastatic malignancy, for the radiographic manifestations were not connected by us with the patient's skin disease. The histologic diagnosis of systemic mastocytosis was also missed primarily, because mast cells are not visualized in the conventional hematoxylin and eosin staining of the histologic sections. Special stainings revealed mastocytosis. Irregular remodeling of the bone was also seen, which was consistent with the radiographically irregular bone structure.