Evaluation of ketotifen in corticosteroid-dependent idiopathic anaphylaxis. To study the possible efficacy of ketotifen (K) in the treatment of idiopathic anaphylaxis (IA), K was administered in an open-label trial to six patients with IA who required corticosteroids at doses below which their disease could not be controlled. During the study, patients continued to receive noncorticosteroid medications (eg, antihistamines, oral adrenergic agents) that had been used on a regular basis for treatment of IA before study, but periodic attempts to reduce corticosteroid doses were made. A reduction of prednisone dose with continued control of disease was judged to be evidence for a beneficial effect of K. After 7 to 16 months of K administration, three patients were judged to have had probable benefits from K (reductions in alternate day prednisone dose requirement from 40 mg to none, 35 mg to 15 mg, and 30 mg to none), and one patient had a possible benefit from K (dose reduction from 100 to 120 mg to 77 mg). Two other patients were unable to tolerate major decreases in prednisone without developing symptoms or signs of IA. Within the limitations of this study design, we conclude that K may be efficacious in the treatment of some patients with IA, and that further trials of K in the treatment of IA are indicated. 