Purpose: Positive expiratory pressure (PEP) breathing has been shown to increase arterial oxygenation during acute hypoxic exposure but the underlying mechanisms and consequences on symptoms during prolonged high-altitude exposure remain to be elucidated. Methods: Twenty-four males (41 ± 16 years) were investigated, at sea level and at 5,085 m after 18 days of trekking from 570 m. Participants breathed through a face-mask with PEP = 0 cmH 2 O (PEP 0 , 0–45 th min) and with PEP = 10 cmH 2 O (PEP 10 , 46–90 th min). Arterial (SpO 2 ), quadriceps and prefrontal (near infrared spectroscopy) oxygenation was measured continuously. Middle cerebral artery blood velocity (MCAv, transcranial Doppler), cardiac function (2D-echocardiography), extravascular lung water accumulation (UsLC, thoracic ultrasound lung comets) and acute mountain sickness (Lake Louise score, LLS) were assessed during PEP 0 and PEP 10 . Results: At 5,085 m with PEP 0 , SpO 2 was 78 ± 4%, UsLC was 8 ± 5 (a.u.) and the LLS was 2.3 ± 1.7 (all P < 0.05 versus sea level). At 5,085 m, PEP 10 increased significantly SpO 2 (+9 ± 5%), quadriceps (+2 ± 2%) and prefrontal cortex (+2 ± 2%) oxygenation ( P < 0.05), and decreased significantly MCAv (−16 ± 14 cm.s –1 ) and cardiac output (−0.7 ± 1.2 L.min –1 ) together with a reduced stroke volume (−9 ± 15 mL, all P < 0.05) and no systemic hypotension. PEP 10 decreased slightly the number of UsLC (−1.4 ± 2.7, P = 0.04) while the incidence of acute mountain sickness (LLS ≥ 3) fell from 42% with PEP 0 to 25% after PEP 10 ( P = 0.043). Conclusion: PEP 10 breathing improved arterial and tissue oxygenation and symptoms of acute mountain sickness after trekking to very high altitude, despite reduced cerebral perfusion and cardiac output. Further studies are required to establish whether PEP-breathing prophylactic mechanisms also occur in participants with more severe acute mountain sickness.