• The effectiveness of plasma exchange in amyotrophic lateral sclerosis (ALS) was tested in four patients. Electromyography (EMG) and clinical examinations were carried out before, during, and after the treatment. The EMG data showed a progressive unaffected motor neuronal decay. Clinically the course of the disease was not dissimilar to that usually encountered, although a momentary improvement was observed in two patients after the first procedure. Plasma exchange appears to be of no value in treating ALS.