Patients with moderate to severe COPD living below 800 m, will be recruited to participate in a randomized cross-over field trial evaluating the hypothesis that exercise capacity during a 4 day sojourn at moderate altitude is reduced in comparison to low altitude. Outcomes will be assessed during 2 days in Zurich (490 m, low altitude baseline), 2 days at Davos Clavadel (1650 m), and 2 days at Davos Jakobshorn (2590 m). The main outcome is the 6 minute walk distance.