BACKGROUND: Humidified high-flow nasal cannula (HFNC) is a novel method of oxygen delivery with increasing use in emergency departments and intensive care settings despite little evidence showing benefit over standard oxygen delivery methods (standard O 2 ). The aim of this study was to determine whether HFNC compared with standard O 2 given to subjects in acute respiratory distress would reduce the need for noninvasive ventilation or invasive ventilation. METHODS: This was a pragmatic open randomized controlled trial in adult subjects with hypoxia and tachypnea presenting to a tertiary academic hospital emergency department. The primary outcome was the need for mechanical ventilation in the emergency department. RESULTS: We screened 1,287 patients, 322 met entry criteria and 19 were excluded from analysis. Of these, 165 randomized to HFNC and 138 to standard O 2 were analyzed. Baseline characteristics were similar. In the HFNC group, 3.6% (95% CI 1.5-7.9%) versus 7.2% (95% CI 3.8 -13%) in the standard O 2 group required mechanical ventilation in the emergency department (P ؍ .16), and 5.5% (95% CI 2.8 -10.2%) in HFNC versus 11.6% (95% CI 7.2-18.1%) in the standard O 2 group required mechanical ventilation within 24 h of admission (P ؍ .053). There was no difference in mortality or stay. Adverse effects were infrequent; however, fewer subjects in the HFNC group had a fall in Glasgow coma score due to CO 2 retention, 0% (95% CI 0 -3%) versus 2.2% (95% CI 0.4 -6%). One in 12 subjects did not tolerate HFNC. CONCLUSIONS: HFNC was not shown to reduce the need for mechanical ventilation in the emergency department for subjects with acute respiratory distress compared with standard O 2 , although it was safe and may reduce the need for escalation of oxygen therapy within the first 24 h of admission.