Malnutrition is a well-known independent survival factor in ALS patients. Total body impedance analysis (BIA) has been validated for the measurement of fat-free mass. Another physical value obtained by BIA is phase angle (PA) that has been proposed to be an index of malnutrition or a survival prognostic factor in several diseases. We measured PA in a sample of 168 ALS patients and analysed the relationships between PA and neurological, respiratory and nutritional status, and determined whether PA is an independent prognostic factor for survival in ALS. BIA was performed using an Analycor3 instrument with surface electrodes at 50 kHz and 800 microA. PA was obtained using the formula: PA =arctan (Xc/R) (in degrees) where Xc is the body reactance and R the bioelectric resistance. Relationships between quantitative variables were assessed by linear correlation coefficients and through univariate and multivariate linear regression analyses. To assess the prognostic role of PA on survival of patients, we performed survival analyses based on the Kaplan-Meier method using the log rank test, and on univariate and multivariate Cox proportional hazard model. Baseline time was time from multidisciplinary visit (T1) to death or censoring time. PA was always significantly lower in ALS patients than in healthy control subjects published in the literature. PA was lower in our ALS malnourished patients than in non-malnourished patients (3.3+/-1.1 degrees versus 2.5+/-0.6 degrees , p =0.003). In multivariate linear regression analysis, manual muscle testing (p<0.0001), vital capacity (p<0.001), fat-free mass (p<0.0001), time between initial signs and T1 (p<0.002) and age (p<0.001) were independently significantly linked with PA and explained 48% of PA variation. The survival of ALS patients from T1 was significantly linked with PA after adjustment on BMI, CV and time between initial signs and multidisciplinary visit, clinical form, age and sex. Patients with a PA <2.5 degrees had a significantly poorer survival rate than patients with a PA >2.5 degrees . We concluded that in ALS patients, PA is greatly decreased. It is related to the nutritional status of patients, and is also an independent prognostic factor of survival.