We investigated the relationship between nasal flaring and SpO2 in 19 patients with Duchenne muscular dystrophy (DMD) and 26 patients with myotonic dystrophy (DM1). In DMD patients, nasal flaring was observed when SpO2 was lower than 96%, while it was not seen even at 82% of SpO2 in DM1. None of the DM1 patients could perform voluntary nasal flaring. Nasal flaring is a useful indicator of hypoxemia in DMD but not in DM1. It remains to be elucidated whether the lack of nasal flaring in DM1 patients is due to abnormal respiratory central mechanism or nasal muscle weakness.