Objective: To investigate the cause and nature of palpitations occurring at high altitude. Methods: Implantable loop recorders were inserted subcutaneously in the left pectoral region of 9 healthy male volunteers. Subjects flew to Kathmandu (1,250 m) and then Lukla (2,800 m) before immediately commencing an identical ascent and descent profile to high altitude. The loop recorders were activated with any episode of palpitations and during exercise, rest and sleep. Arterial oxygen saturation was assessed concomitant with device activation. Results: Above 5,000 m all subjects reported palpitations during exercise. All subjects demonstrated sinus tachycardia and marked sinus arrhythmia; one subject demonstrated atrial flutter; one subject had non-conducted p waves, and a further subject had marked ST segment depression. Conclusions: Significant arrhythmias occur at high altitude. In view of the increased risk of sudden cardiac death at high altitude, and considering that the elderly account for 15% of the 100 million visitors to altitude annually, further investigation is required.