Aim: Early identification of patients likely to require ventilatory support is a key emergency department priority. Bedside diaphragm ultrasound provides a rapid assessment of respiratory muscle function, and the diaphragm thickening fraction quantifies diaphragmatic contraction during tidal breathing. Material and methods: In this prospective observational pilot study, adults with acute respiratory failure in a single emergency department underwent diaphragm ultrasound within 6 hours of presentation. The primary outcome was initiation of non-invasive ventilation or invasive mechanical ventilation from emergency department presentation through hospitalization. Results: Among 56 patients, 22 (39%) required ventilatory support. Diaphragm thickening fraction was lower in those requiring support (median 20.93% vs 29.8%; p = 0.006). Higher diaphragm thickening fraction was associated with lower odds of ventilatory support (OR 0.93 per 1% increase, 95% CI 0.88–0.98; p = 0.008). Discriminative ability was moderate (AUC 0.72, 95% CI 0.58–0.87), and the optimal cutoff was 22.1% (sensitivity 59%, specificity 71%). Conclusions: Early emergency department assessment of diaphragm thickening fraction may help identify patients at increased risk of requiring ventilatory support.