Chronic urticaria, perhaps more than other dermatoses, challenges the physician and patient to find its cause. At times this challenge is met with ease, e.g., in the patient with dermographism, a penicillin reaction, cold allergy, or mastocytosis. Yet at other times the hives remain completely mysterious and capricious despite a wearying series of trial diets, environmental changes, skin tests, and psychotherapy. The causes of urticaria seem legion, and in some patients the elimination of 1 etiologic suspect may be followed by the appearance of 10 new ones. We are therefore prompted to record our studies of a patient with chronic urticaria due to a hypersensitivity so extreme as to enable us to view the disease, as it were, under a "clinical microscope." His exquisite sensitivity permitted us to discern a common causal thread extending throughout his entire history of hives. Thus, it was possible to observe a single basic cause,