Machu Picchu is located at an altitude of 7,972 feet (2,430 meters) above sea level. Huayna Picchu, the peak that looms over the archeological site, rises to a height of 8,920 feet (2,720 meters) above sea level.
At these heights, travelers -- no matter how physically fit -- may experience symptoms of altitude sickness. Acute mountain sickness (AMS) typically occurs at heights of 8,000 feet (2,500m) and above, placing Machu Picchu right on the cusp in terms of risk.
Travelers, therefore, should exercise greater caution upon arrival in the city of Cusco, the gateway to Machu Picchu. Cusco is located at an elevation of 11,152 feet (3,399m) above sea level, significantly higher than the Inca citadel. When you arrive in Cusco, particularly if you have flown in directly from Lima (at sea level), you should try to set aside at least 24 hours to acclimatize.
New arrivals also have the option of moving from Cusco to nearby towns located in the Sacred Valley. These towns are at slightly lower altitudes, providing a more gentle form of acclimatization before moving back to Cusco. Once you have acclimatized in Cusco or in the Sacred Valley, you should have no problems with the altitude at Machu Picchu itself. You may still feel breathless while walking around the site, but the risk of altitude sickness will be minimal.