The factors limiting exercise performance at extreme altitude remain unclear. Maximal exercise tests have been performed up to 7400 metres on small numbers of climbers using Douglas bag expired gas analysis. However, to our knowledge a fully validated breath by breath cardiopulmonary exercise testing system has not been previously used at extreme altitude. Using hypobaric chambers, cold chambers and field testing, we have validated a portable breath by breath expired gas analysis system which we plan to use on Mount Everest in May 2007. The system has been shown to be reliable up to altitudes of 9000 metres and at temperatures of -20 degrees celcius. On Everest we will measure changes in anaerobic threshold, ventilatory equivalents for oxygen and carbon dioxide and maximal oxygen consumption in a group of 15 climbers during a standardised ascent from sea level to 8000 metres. Pilot data from a field expedition to Tibet demonstrate that the anaerobic threshold can be reliably identified at altitude. We hope to present this pilot data along with early results from Everest.