Hemoglobin mass is a key factor for maximal exercise capacity. Some athletes apply prohibited techniques and substances with intent to increase hemoglobin mass and physical performance, and this is often difficult to prove directly. Autologous red blood cell transfusion cannot be traced on reinfusion, and also recombinant erythropoietic proteins are detectable only within a certain timeframe. Novel erythropoietic substances, such as mimetics of erythropoietin (Epo) and activators of the Epo gene, may soon enter the sports scene. In addition, Epo gene transfer maneuvers are imaginable. Effective since December 2009, the World Anti-Doping Agency has therefore implemented "Athlete Biologic Passport Operating Guidelines," which are based on the monitoring of several parameters for mature red blood cells and reticulocytes. Blood doping may be assumed, when these parameters change in a nonphysiologic way. Hematologists should be familiar with blood doping practices as they may play an important role in evaluating blood profiles of athletes with respect to manipulations, as contrasted with the established diagnosis of clinical disorders and genetic variations.