The clinical diagnosis of amyotrophic lateral sclerosis was identified in 668 patients who were followed up for periods up to three years. This disease was found to be more common in women than previously believed. The five-year survival for all patients was found to be 39.4%, but younger patients had a substantially better prognosis than those in whom the disease was diagnosed when over the age of 50 years. When the mode of onset was considered, the spinal form was associated with a threefold better five-year survival than the bulbar form.