The proliferation of mast cells in the skin of patients with urticaria pigmentosa has been well described.*We would present the findings in systemic mast-cell disease suggesting that the disseminate variety of disease may be more frequent than has been previously recognized. Further, some ideas regarding the mast cell and its function are listed to explain certain of the findings here noted. The possible action of the mast cell in the inflammatory response is also considered. I. SYSTEMIC MAST-CELL DISEASE Urticaria pigmentosa ordinarily occurs in childhood, with approximately 70% of the total recorded cases1having their onset in the first year. The characteristic cutaneous lesions are macules, papules, or, less frequently, nodules which vary from yellow-brown to violet. The tendency of these lesions when irritated to urticate is the basis for the name of the disease. The trunk is ordinarily the most extensively