Previous studies have supported non-invasive ventilation (NIV) in amyotrophic lateral sclerosis (ALS), indicating that it prolongs survival. However, criteria for the use of NIV are yet to be defined. The aim of the present study was to evaluate the potential use of nocturnal pulse oximetry (NPO) as a tool for determining the most appropriate time at which to initiate NIV in ALS patients. We conducted a prospective, comparative, historical-controlled clinical study of 64 consecutive ALS patients. Group 1 (historical control group) comprised 44 patients, 14 women and 30 men, with a mean age of 60+/-13 years, in whom we used NIV after detecting early signs of diurnal respiratory insufficiency (RI). Group 2 had 20 ALS patients, 8 women and 12 men, with a mean age of 56+/-11 years, for whom we started NIV after detecting more than 15 periods of nocturnal desaturation/hour by NPO. All patients were periodically evaluated with Norris spinal and bulbar scores, respiratory function tests (RFT) and NPO at three month intervals for one year. They were subsequently followed until death or loss of autonomy from the ventilator. We compared survival time or time to loss of autonomy from the ventilator between both groups. Survival was longer in Group 2 (p<0.002). We concluded that NPO is a valuable screening test to establish the need for NIV. Our results also suggest that the early diagnosis of RI and the early use of NIV increase patient compliance with NIV.