Aerobic capacity and performance are associated with increased blood fluidity, while sedentarity leads to decreased exercise performance, and blood hyperviscosity. We aimed at investigating the relationships among body composition, blood rheology and exercise performance in this situation. In 46 sedentary subjects (53.09 ± 1.79 yr old; BMI = 32,35 ± 0,80) attending our unit for an exercise prescription we performed an exercise test to assess aerobic capacity, together with blood lipid profile and blood viscosity (MT 90 viscometer, Myrenne erythroaggregometer). The maximal aerobic capacity VO2max was not correlated to blood rheology but its changes were negatively correlated to those of plasma viscosity (r = -0.679) and pre-training VO2max values were negatively correlated to the BMI (r = -0.45873; p = 0.00430) and fatness (waist circumference r = -0.53476; p = 0.00406). Hemorheological parameters were as expected correlated to blood lipids. The main determinant of the RBC rigidity index Tk was HDL-cholesterol (r = -0.70026; p = 0.00121). The main determinant of M1 is HDL-cholesterol (r = -0.5157; p = 0.0238). RBC aggregability "M" is negatively correlated to total cholesterol (r = -0.758932; p = 0.000105); HDL-cholesterol (r = -0.62232; p = 0.00444); LDL-cholesterol (r = -0.64486; p = 0.00386). Whole blood viscosity is correlated to triglycerides (r = 0.8569; p = 0.00000140) and negatively correlated to HDL-cholesterol (r = -0.5622; p = 0.0122). Waist circumference (an index of abdominal fatness) is correlated to blood viscosity (r = 0.597; p = 0.00888). The waist to hip ratio is correlated to Hct (r = 0.70075 p = 0.00120) and to blood viscosity (r = 0.8124334; p = 0.0000420). Fat-free mass is correlated to blood viscosity (r = 0.66528; p = 0.00137) and hematocrit (r = 0.64350; p = 0.00220). Hip circumference is negatively correlated to plasma viscosity (r = -0.5007; p = 0.0290). Therefore, this study confirms that hemorheological parameters are influenced by blood lipids, that changes in plasma viscosity are correlated to those of aerobic capacity, and that abdominal fat mass is associated with a worsening of blood rheology and of exercise performance. By contrast, gluteal fat (a factor associated with favorable lipid profile and high insulin sensitivity) is associated with a decrease in plasma viscosity, and fat-free mass is associated with higher blood viscosity and hematocrit, consistent with recent literature linking its size in abdominal obesity with a deleterious metabolic profile.