Patients with pulmonary hypertension or with interstitial lung disease living below 800 m will be invited to participate in a randomized, cross-over field trial evaluating the hypotheses that: a), exercise capacity during a 2 day sojourn at moderate altitude is reduced in comparison to low altitude; b), exercise capacity during a 2 day sojourn at moderate altitude is improved by nocturnal oxygen therapy compared to room air (sham oxygen) administered during nights at altitude. Outcomes will be assessed at low altitude (Zurich, 490 m, baseline), and during 2 study periods of 2 days each spent at moderate altitude (St. Moritz Salastrains, 2048 m), separated by a wash-out period of at least 2 weeks spent at low altitude (<800 m). The order of stays at the different altitudes and of the treatments will be randomized.