High-altitude pulmonary hypertension (HAPH) is an altitude-related illness associated with hypoxaemia that may promote sympathetic excitation and prolongation of the QT interval. The present case-control study tests whether QT intervals, markers of malignant cardiac arrhythmias, are prolonged in highlanders with HAPH (HAPH+) compared to healthy highlanders (HH) and healthy lowlanders (LL). The mean pulmonary artery pressure (mPAP) was measured by echocardiography in 18 HAPH+ (mPAP, 34 mmHg) and 18 HH (mPAP, 23 mmHg) at 3,250 m, and 18 LL (mPAP, 18 mmHg) at 760 m, Kyrgyzstan (p .05 all mPAP comparisons). Groups were matched for age, sex and body mass index. Electrocardiography and pulse oximetry were continuously recorded during nocturnal polysomnography. The heart rate-adjusted QT interval, QTc, was averaged over consecutive 1-min periods. Overall, a total of 26,855 averaged 1-min beat-by-beat periods were semi-automatically analysed. In HAPH+, maximum nocturnal QTc was longer during sleep (median 456 ms) than wakefulness (432 ms, p .05) and exceeded corresponding values in HH (437 and 419 ms) and LL (430 and 406 ms), p .05, respectively. The duration of night-time QTc440 ms was longer in HAPH+ (median 144 min) than HH and LL (46 and 14 min, p .05, respectively). HAPH+ had higher night-time heart rate (median 78 beats/min) than HH and LL (66 and 65 beats/min, p .05, respectively), lower mean nocturnal oxygen saturation than LL (88% versus 95%, p .05) and more cyclic oxygen desaturations (median 24/hr) than HH and LL (13 and 3/hr, p .05, respectively). In conclusion, HAPH was associated with higher night-time heart rate, hypoxaemia and longer QTc versus HH and LL, and may represent a substrate for increased risk of malignant cardiac arrhythmias.