I m vacationing near Phoenix AZ. I drove to Montezuma s Castle, high altitude, about 6000 feet and started experiencing pain similar to cramps on the high side of my left calf. It would hit sharply, not a muscle tightening but about 75% of the pain of a muscle cramp and slowly diminish, go away after 3 to 5 minutes but then 5 or 10 minutes later, come back again. This continued but when I returned to a lower altitude, went away. The next day I drove to the south rim of the Grand Canyon and on the way, when I got to high altitude it came back again. I stayed overnight in Flagstaff, 6 to 7000 feet and it continued overnight. When I returned to the lower altitude the next day it went away and has not come back. I remain at low altitude. I m convinced it has to be altitude related, but is this a symptom of anything serious. I m a 67 year old man and in good shape and as far as I know have no medical problems other than some occasional lower back problems that don t keep me from doing anything.