A relationship exists between exposure to high altitude and increased coagulability. Mean platelet volume is a parameter of platelet functions and may be a marker for increased platelet aggregability. The aim of this study was to compare the mean platelet volumes and platelet counts in patients who experienced an acute coronary event at moderately high altitude and at sea altitude. Four hundred and one patients who experienced an acute coronary event were enrolled, of them 211 were born and had been living at the sea level, while 190 were born and had been living at high altitude (at least 2,000 m above the sea level). Patients were compared regarding the mean platelet volumes and platelet counts. The mean platelet volumes were significantly higher in patients living in high altitude (P = 0,001). No statistically significant differences were found among the groups regarding the platelet counts. As a result, this increased MPV values in highlanders who experienced an acute coronary event may reflect increased platelet aggregability.