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The capacity for ketotifen to prevent mast-cell degranulation in vivo was studied in patients with physical urticarias. Patients were exposed to the appropriate stimulus to elicit their physical urticaria before and during ketotifen therapy. Histamine concentrations in plasma samples, obtained before and serially after the physical provocation, were determined by radioenzymatic thin-layer chromatography. Ketotifen therapy was associated with marked reductions in plasma histamine levels after stimulation and in clinical evidence of urticaria in each patient. A direct correlation of ketotifen therapy and a reduction in histamine release was confirmed in a patient with a cold-induced urticaria who was studied again after discontinuation and again after reinstitution of therapy. Although the mechanism of action is unknown, this report shows that ketotifen is capable of inhibiting cutaneous mast-cell degranulation and its accompanying symptoms. These findings suggest important therapeutic alternatives for patients with mast-cell-mediated diseases.