Between 10 percent and 20 percent of amyotrophic lateral sclerosis cases are familial (FALS). The announcement of a diagnosis of ALS to a patient having relatives already affected with this disease must take into account the familial history and should focus on the improvement in management of ALS patients. Diagnosis of FALS implicitly raises the notion of genetic factors and the possibility for an heritability of the disease (risk of ALS for relatives). Although it is possible to search for mutations in SOD1 gene in ALS patients and asymptomatic relatives, the incomplete penetrance of the disease, the low percentage (10 to 20 percent) of FALS linked to SOD1 mutation and the existence of non causal SOD1 mutations complicate the conclusions of genetic investigations concerning the real risk for a relative with a SOD1 mutation to develop ALS.