To the editor: Plasma exchange has become an accepted form of treatment for myasthenia gravis since Pinching and Peters first reported this approach in 1976 (1). Several studies have suggested that the clinical improvement after plasma exchange is due to a drop in circulating antireceptor antibody (2, 3). We recently treated by plasma exchange a myasthenic patient in whom a significant decline in antireceptor antibody was not accompanied by any clinical improvement. A 37-year-old woman had been treated for myasthenia for 10 years. Early in the course of her disease, a malignant thymoma had been removed via trans-sternal approach, and