ObjectiveTo compare the efficacy of continuous positive airway pressure (CPAP) versus usual care for prehospital patients with severe respiratory distress.MethodsWe conducted a parallel group, individual patient, non-blinded randomised controlled trial in Western Australia between March 2016 and December 2018. Eligible patients were aged ≥40 years with acute severe respiratory distress of non-traumatic origin and unresponsive to initial treatments by emergency medical service (EMS) paramedics. Patients were randomised (1:1) to usual care or usual care plus CPAP. The primary outcomes were change in dyspnoea score and change in RR at ED arrival, and hospital length of stay.Results708 patients were randomly assigned (opaque sealed envelope) to usual care (n=346) or CPAP (n=362). Compared with usual care, patients randomised to CPAP had a greater reduction in dyspnoea scores (usual care −1.0, IQR −3.0 to 0.0 vs CPAP −3.5, IQR −5.2 to −2.0), median difference −2.0 (95% CI −2.5 to −1.6); and RR (usual care −4.0, IQR −9.0 to 0.0 min^-1 vs CPAP −8.0, IQR −14.0 to −4.0 min^-1), median difference −4.0 (95% CI −5.0 to −4.0) min^-1. There was no difference in hospital length of stay (usual care 4.2, IQR 2.1 to 7.8 days vs CPAP 4.8, IQR 2.5 to 7.9 days) for the n=624 cases admitted to hospital, median difference 0.36 (95% CI −0.17 to 0.90).ConclusionsThe use of prehospital CPAP by EMS paramedics reduced dyspnoea and tachypnoea in patients with acute respiratory distress but did not impact hospital length of stay.Trial registration numberACTRN12615001180505.