['Non-intubated critically ill patients are often treated by high-flow oxygen for acute respiratory failure. There is no current recommendation for humidification of oxygen devices.', 'We conducted a prospective randomized trial with a final crossover period to compare nasal airway caliber and respiratory comfort in patients with acute hypoxemic respiratory failure receiving either standard oxygen therapy with no humidification or heated and humidified high-flow oxygen therapy (HHFO₂) in a medical ICU. Nasal airway caliber was measured using acoustic rhinometry at baseline, after 4 and 24 hours (H4 and H24), and 4 hours after crossover (H28). Dryness of the nose, mouth, and throat was auto-evaluated and assessed blindly by an otorhinolaryngologist. After the crossover, the subjects were asked which system they preferred.', 'Thirty subjects completed the protocol and were analyzed. Baseline median oxygen flow was 9 and 12 L/min in the standard and HHFO₂ groups, respectively (P = .21). Acoustic rhinometry measurements showed no difference between the 2 systems. The dryness score was significantly lower in the HHFO₂ group at H4 (2 vs 6, P = .007) and H24 (0 vs 8, P = .004). During the crossover period, dryness increased promptly after switching to standard oxygen and decreased after switching to HHFO₂ (P = .008). Sixteen subjects (53%) preferred HHFO₂ (P = .01), especially those who required the highest flow of oxygen at admission (P = .05).'] Does heated and humidified high-flow oxygen therapy reduce discomfort during hypoxemic respiratory failure?