BACKGROUND Various breathing abnormalities (Neurology 2009; 73: 1218) have been proposed as indicators for the introduction of non-invasive positive-pressure ventilation (NIV) in patients with amyotrophic lateral sclerosis (ALS). We were interested in the usefulness of symptoms of respiratory insufficiency and abnormal results of daytime arterial gas analyses (AGA) as predictors of survival and the effect of NIV on respiratory volumes and pressures. METHODS Reported symptoms, respiratory subscore of the ALS Functional Rating Scale (ALSFRS-r), Norris scale (Norris-r), and AGA were retrospectively analyzed in 189 ALS patients. Longitudinal follow-up of forced vital capacity (FVC), maximal inspiratory and expiratory pressure (MIP, MEP), and sniff nasal pressure (SNP) were analyzed with regard to the introduction of NIV. RESULTS Respiratory symptoms were a bad prognostic sign (P = 0.007). Abnormalities in Norris-r, ALSFRS-r, pO(2), pCO(2), and oxygen saturation tended to be associated with a shorter survival, although they were not statistically significant. NIV prolonged survival and reduced the decline in FVC (P = 0.007), MIP, MEP, and SNP (the last three were not statistically significant). Symptoms, abnormal FVC, and AGA do not always coincide, and they can appear in a different sequence. CONCLUSIONS Any respiratory abnormality should prompt the clinician to start discussing NIV with the patient. NIV prolongs survival and improves respiratory function.