Abstract NIV adherence ('quantity' of ventilation) has a prognostic impact in amyotrophic lateral sclerosis (ALS). We hypothesized that NIV effectiveness ('quality') could also have a similar impact. NIV effectiveness was evaluated in 82 patients within the first month (M1) and every three months (symptoms, arterial blood bases, and nocturnal pulsed oxygen saturation - SpO2). Kaplan-Meier survival and risk factors for mortality one year after NIV initiation were evaluated. Forty patients were considered 'correctly ventilated' at M1 (Group 1, less than 5% of nocturnal oximetry time with an SpO2<90% - TS90) while 42 were not (Group 2). Both groups were comparable in terms of respiratory and neurological baseline characteristics. Survival was better in Group 1 (75% survival at 12 months) than in Group 2 (43% survival at 12 months, p = 0.002). In 12 Group 2 patients corrective measures were efficient in correcting TS90 at six months. In this subgroup, one-year mortality was not different from that in Group 1. Multivariate analysis identified independent mortality risk factors expectedly including bulbar involvement (HR = 4.31 (1.73 - 10.76), p = 0.002), 'rapid respiratory decline' (HR = 3.55 (1.29 - 9.75), p = 0.014) and vital capacity (HR = 0.97 (0.95 - 0.99), p = 0.010), but also inadequate ventilation in the first month (HR = 2.32 (1.09 - 4.94), p = 0.029). In conclusion, in ALS patients NIV effectiveness to correct nocturnal desaturations is an independent prognostic factor.