 Transcutaneous PCO2 tcPCO2 is an attractive technique to evaluate non invasively nocturnal hypoventilation. The technique is well validated in different settings. Its use in neuromuscular disorders NMD is recent. In particular one study has demonstrated a high predictive value of tcPCO2 for the development of daytime hypoventilation within 1 year. To our knowledge, this technique has not been specifically assessed in ALS. There is a potential role for nocturnal PtcCO2 monitoring in the close follow up of ALS patients. Indeed, a close respiratory follow up of ALS patients is essential to determine the optimal timing of NIV, avoiding the occurence of unexpected acute respiratory failure. 