• A 2½-year-old girl presented with monthly episodes of angioneurotic edema, eruption of pruritic papules, and fever. During acute episodes, white blood cell counts rose as high as 52,100/cu mm with 62% eosinophils, and body weights increased up to 20% of remission weight. Short courses of prednisone acetate caused rapid defervescence, resolution of angioneurotic edema, and lowering of eosinophil counts. In a one-year follow-up no evidence was found for cardiac or other visceral organ involvement. Findings of extensive diagnostic evaluations revealed no evidence for atopy, neoplasm, collagen-vascular disease, or parasitic infestation. Results of immunologic studies were essentially normal with the exception that this patient had a high level of circulating activated helper T cells. Biopsy specimens of the skin lesions revealed dermal infiltration of lymphocytes and eosinophils with deposition of eosinophil major basic protein in the extracellular matrix. Awareness of this clinical entity and its distinction from the hypereosinophilic syndrome is important because of its favorable prognosis and rapid response to corticosteroid therapy.