Urticaria pigmentosa presenting with massive peripheral eosinophilia.

A 5-month-old Hispanic boy had a diffuse eruption and massive peripheral eosinophilia. Mastocytosis was diagnosed by skin biopsy and Giemsa stain. Other causes of eosinophilia were ruled out by bone marrow examination and negative cultures. The patient's course improved with antihistamines and the removal of mast cell degranulating agents. Clinicians need to be aware that mastocytosis should be considered in the differential diagnosis of eosinophilia.