A young woman developed myositis, myocarditis, and vasculitis associated with severe eosinophilia following the use of sulfisoxazole (Gantrisin). Her illness was not altered by corticosteroid therapy and appeared to be progressive. Following leukopheresis and removal of large numbers of eosinophils on a continuous flow cell separator, there was a prompt decrease in the eosinophilia and rapid clinical improvement. Although the mechanism of this effect is unknown, leukopheresis may be a worthwhile modality of therapy in other patients with progressive hypereosinophilic disorders. (JAMA 230:1004-1005, 1974)