CASE PRESENTATION: A 34-year-old male highlander living at 2200 meters altitude was hospitalized at West China Hospital, Chengdu. No specific findings were obtained except for polycythemia (hemoglobin 226 g/L). As heavy snoring and witnessed apneas, a standard in-lab polysomnography were performed. PSG showed apnea-hypopnea index (AHI) was 28.6/ h with a mean duration of 1.22 min. The longest OSA event was 6.3 min (Figure 1 A, B). An overnight continuous positive airway pressure (CPAP) titration was performed. His AHI reduced to 3.7/ h, with optimal pressure of 13 cm H2O. Ten days later after he returned to high altitude, an overnight at home PSG recording (Figure 1 C) showed that shorter duration of the longest (6.3 min to 3.5 min) and averaged (1.22 min to 0.8 min) OSA events, as well as higher AHI (28.6/ h to 60/ h) as compared to at sea level. The rest of follow-up examinations were carried out in West China Hospital. After one month CPAP therapy at high altitude, his headache and dizziness were completely remitted. At 2nd and 3rd month (Figure 1 D) follow-ups, overnight PSG recordings showed that the longest duration of OSA events were shortened to 3.8 and 3.5 min, and mean duration reduced to 0.9 and 0.82 min at sea level, respectively. This patient’s hemoglobin level was parallel decreased with shortened duration of the longest OSA events (Figure 1 E).