It is well known that body composition influences blood rheology (higher blood viscosity and RBC aggregation in obese individuals). On the other hand, blood rheology is related to exercise performance (the fitter the athlete the more fluid the blood). The 'paradox of hematocrit' is that most of the time trained athletes have a lower hematocrit while doping aims at increasing it, a situation which seems to challenge physiology and can be explained by the fact that systemic hematocrit may have poor physiological relevance at the microcirculatory level in exercising muscles. However, recent studies dealing with the marked differences among hemorheologic profiles observed in selected sports lead to the concept that the relationships between hemorheology and performance on one hand and body composition and hemorheology on the other hand are more complex and somewhat different according to the variety of exercise and the physiological and/or pathophysiological background. Increased fat mass, but also fat free mass independently of fat mass, are both correlated to impairments of blood fluidity, consistent with the emerging concept that beside increases in fat mass, increases in fat free mass may also be related to metabolic and circulatory disturbances.