Endurance athletes use several approaches and modalities for “naturally” increasing the aerobic performances, including intermittent hypoxic which refers to the discontinuous use of normobaric or hypobaric hypoxia, in an attempt to reproduce some of the key features of altitude acclimatization, with the ultimate goal to improve sea-level athletic performance (Levine 2002). We read with interest the article of Sanchis-Gomar et al. (2009) on the eVect of intermittent hypoxia on hematological parameters after recombinant human erythropoietin (rHuEpo) administration. The Wndings of this valuable study further highlight the fuzzy boundary existing between what is licit and what is not in sports. Basically, the authors showed that intermittent hypoxic treatment is at least as eVective as the administration of rHuEpo to increase the red blood cell mass and, inherently, the aerobic performances. After 23 days, rHuEPO administration, normoxic animals showed a 45% decrease in their hemoglobin concentration when compared with the animals that underwent a 12-h period of hypoxia daily. Likewise, the normoxic animals showed a 35% decrease in their hematocrit concentration when compared with the animals that underwent 12-h periods of hypoxia daily (Sanchis-Gomar et al. 2009). Although this experiment was carried on in mice, similar Wndings were previously observed in humans (Heinicke et al. 2003). The crucial question is, therefore, should intermittent hypoxic training be considered a form of blood doping, such as the administration of erythropoiesis-stimulating substances, blood transfusions and altitude training simulator systems? There is no deWnitive answer to this question, but the problem is tangible. Blood transfusions (Lippi et al. 2006) and altitude training simulator systems (Lippi et al. 2007) have been banned by the World Anti-Doping Agency (WADA) mainly due to the potential adverse cardiovascular outcomes following a substantial increase in red blood cell mass and blood viscosity. rHuEPO has been banned because it produces the same cardiovascular eVects (Lippi and Guidi 2000), but it is also associated with a serious risk of venous thrombosis (Dicato 2008) and development of the life-threatening epoetin-associated pure red cell aplasia (McKoy et al. 2008). Therefore, if altitude training simulator systems and blood doping are to be banned for the risk of adverse cardiovascular outcomes, why should not be “natural” intermittent hypoxic training at high altitude, since it produces nearly the same eVects on erythropoiesis? To corroborate further our doubts is the evidence of additional unfavorable biochemical changes associated with severe intermittent hypoxia, including decreased antioxidative capacity and increased lipid peroxidation, which would led to suppression of vascular endothelial function and cause impairment of vascular hemodynamics (Wang et al. 2007).