In vitro studies seem to indicate that a serum factor may be involved in the pathogenesis of motor neuron disease. If so, plasmaphoresis might influence the course of amyotrophic lateral sclerosis (ALS) favorably. In the present study, therefore, ALS patients were subjected to weekly 21 plasma exchanges, using a Haemonetics blood separator. Seven other ALS patients, matched as closely as possible with the treatment group regarding age, sex, duration of symptoms as well as degree of involvement, served as a control group. The progression of the disease was followed by an arbitrary grading system, assessment of muscular power by Zadig's dynamometer, and by tests for motor speed, coordination and for pulmonary function. Duration of treatment was from 6 to 15 months. Monthly evaluations indicated that the rate of deterioration was approximately the same in treatment and control groups. Plasmaphoresis carried out in this way does thus not alter the downhill course of ALS.