A 19-month-old boy was found to have atypical diffuse cutaneous mastocytosis. Although severe dermographism and pressure urticaria were present from early infancy, his disease remained undiagnosed because he lacked the characteristic bullous and pigmented lesions of urticaria pigmentosa and the pebbly skin texture usually found in diffuse cutaneous mastocytosis. A skin biopsy from clinically normal skin disclosed a dense infiltrate of mast cells in the upper dermis. Since involvement of other organ systems is frequent in this form of mastocytosis, the patient was studied for internal lesions but none was found at this time. Parents of affected children should be informed that trauma to the skin such as brisk toweling after a bath, and drugs such as aspirin, codeine, and polymyxin B sulfate may precipitate episodes of histamine shock due to degranulation of the mast cells.