Twenty-five cases of amyotrophic lateral sclerosis occurring before the age of 40, collected between 1963 and 1973 in the Neurological Hospital, Lyons, have prompted the authors to make the following observations. -The incidence of disease seems to have been on the increase during this period both in absolute terms and relatively speaking (relative to the total number of patients hospitalized and to the total number of patients hospitalized for amyotrophic lateral sclerosis after the age of forty). -The 25 cases involved mostly women--the opposite of what was found in cases of amyotrophic lateral sclerosis occurring after forty. -Only one was possibly familial. -The onset is mostly with purely pyramidal features or of the topographically circumscribed peripheral type. Development appears to take longer than the average for amyotrophic lateral sclerosis. It would be worth while confirming these findings by analysis of other similar groups.