High altitude produces increased pulmonary capillary pressure by hypoxia induced pulmonary vasoconstriction. It is also possible that hypoxia results in mildly elevated left ventricular (LV) filling pressures that may contribute to the elevated capillary pressures. This study investigates the impact of simulated high altitude on global and regional echocardiographic measures of LV performance and filling pressure. Seventeen healthy individuals underwent transthoracic echocardiography, including tissue Doppler of the septal mitral annulus and basal segments before and after an 18-h overnight stay in a high altitude simulation tent with a FiO(2) of 12%, simulating an altitude of approximately 4,000 m above sea level. In simulated high altitude, the ratio of early transmitral flow velocity to early myocardial relaxation velocity increased 22%, P < 0.001, and the Index of Myocardial Performance increased 30%, P < 0.01 due to an 58% increase in the isovolumic relaxation time (IVRT), P < 0.001. Simulated high altitude leads to a reduction in LV performance with an accompanying increase in markers of LV filling pressure. The significant changes in filling pattern and IVRT in the setting of normal and unchanged systolic function, indicates that hypoxia induces mild diastolic dysfunction in young healthy individuals.