Functional evaluation of athletes and the control of performance, undoubtedly present a complex task, even more so when we need to compare their capacities in unique conditions such as high altitude, since the athlete himself is an individual ensemble of physiological responses. In order to obtain results of a functional evaluation, we studied the performance of altitude acclimatized and non-acclimatized professional soccer players at two levels of altitude (Brutsaert et al., 2000). In the present study, the exercise response during cycle ergometry and the rate of football match energy expenditure (RFE) were measured in two groups of professional players at high altitude (3,600 m) and close to sea level (420 m): Subjects either resided at high altitude and were therefore altitude acclimatized (n=9) (HA), or they were residents of low altitude and non-acclimatized to high altitude (n=11) (LA). Both study groups showed a large decrement in the RFE (0.187 kcal/kg/min , or a 16% decrease) and of peak oxygen consumption (VO2peak) at altitude (10.78 ml/kg/min for LA and 6.27 ml/kg/min for HA). This VO2peak decrement was larger in LA vs. HA (20% vs.13%). LA players also showed higher ventilatory equivalents for oxygen , lower arterial oxygen saturations, and higher lactate concentrations during submaximal exercise. These differences, however, between HA and LA players when being examined at high altitude, were not statistically significant. In addition, all subjects were measured using standard anthropometric techniques, pulmonary function was assessed by measuring minute ventilation (VE), ventilatory response to CO2, forced vital capacity (FVC), index of bronchial permeability (FVC/ FEV1).