Hypobaric hypoxia imposes a catabolic reaction on the human organism. We postulated that there would be a significant reduction in left ventricular (LV) myocardial mass, significant changes in cardiac enzymes (CK-MB, Troponin-T, pro-BNP) and signs of diastolic dysfunction after exposure to hypobaric hypoxia.We studied healthy expedition members (n=7) before departure to (pre), directly after return from (post) and 1 month after return from (1 month follow-up) expeditions to 8000-meter peaks in Himalayas. We used cardiac magnetic resonance (MR) scanning to assess myocardial mass, dual x-ray analysis (DEXA) scan to examine body composition, and blood samples to examine cardiac enzymes and hematological parameters.We found a significant 11 % reduction in mean myocardial mass from pre to post values (p<0,05). Myocardial mass increased significantly from post to 1 month follow up, but does not reach the initial values (pre). Left ventricular end diastolic volum (LVEDV) and stroke volume (LVSV) showed a significant reduction from pre to post (p<0,05), with no signs of normalization at 1 month follow-up examination (p<0,05 from pre to 1 month follow-up). Total lean mass (3,8 % mean reduction), total body weight (5,4 % mean reduction) and total leg fat mass (14% mean reduction) were significantly reduced from pre to post (p<0,05). Except from a slight but significant increase in pro-BNP from pre to post (p<0,05), we found no hematological signs of myocardial dysfunction.The causes for the reduction in myocardial mass remain unknown. We have shown that although myocardial mass is increased 1 month after return to sea level, the remaining decrease in LVEDV and LVSV points towards a diastolic dysfunction that outlasts the hypoxic stimulus.