A retrospective study of endogenous and exogenous risk factors in amyotrophic lateral sclerosis (ALS) was conducted in 35 ALS patients and 35 healthy controls, using guided questioning. No significant correlation was detected between the disease and most of the environmental factors explored. A significant difference (p less than 0.05) was found between patients and controls in only two parameters: 1) occurrence of injuries during the year that preceded the diagnosis of ALS (usually localized to the limbs), and 2) regular practice of sports in adulthood. It appeared that injuries often revealed but could not be considered as a possible cause of the disease. Analysis of the sport activities was not sufficient for a definite conclusion. On the other hand, 11 p. 100 of ALS patients had a family history of ALS, and 25 p. 100 (as against 8 p. 100 of controls) had a history of degenerative disease of the nervous system. These results should lead to a reconsideration of genetic factors or to a search for exposure of several members of the same family to toxic or infectious environmental factors.