Empirical treatment of patients with amyotrophic lateral sclerosis suggested that some modification of the classical downhill course occurred in nonfamilial cases during administration of guanidine hydrochloride in dosages of at least 10 mg per kilogram per day for three months or more. A controlled study is reported in 24 patients assigned randomly to guanidine doses of 2 or 25 mg per kilogram per day. By six months the group receiving the larger dose had more stable disease and lower mortality, but the number of stable cases lost significance by 10 months and no patient showed any improvement. Any short-term benefit in ALS from the administration of guanidine must be balanced against possible major side effects, including acute paralysis and bone marrow depression. Also, a relatively benign course without any treatment is more likely than was thought in the past.