Hemorheologic effects of exercise training ("hemorheologic fitness") are very different according to the mode and the intensity of this training. We previously reported that low intensity endurance training in sedentary patients suffering from the metabolic syndrome sumultaneously improved blood rheology, body composition and lipid oxidation at exercise. We aimed at analyzing the link among these improvements in 24 patients submitted to a 2 months targeted training designed for increasing exercise lipid oxidation. Variations of whole blood viscosity at high shear rate (etab 1000 s(-1)) were explained here by two statistically independent determinants: hematocrit and red cell rigidity. etab decreased in 16 subjects, but increased in 8, due to a rise in hematocrit. Changes in RBC rigidity appeared to reflect weight loss and decrease in LDL cholesterol. Plasma viscosity was related to cholesterol and its training-induced changes are related to those of VO2 max ) but not to lipid oxidation. Red cell aggregability (Myrenne) reflected both the circulating lipids (Chol, HDL and LDL) and the ability to oxidize lipids at exercise. Factors associated to a post-training decrease in aggregability (M1) were weight loss and more precisely decrease in fat mass, improvement in lipid oxidation, rise in HDL-Chol, and decrease in fibrinogen. On the whole the major determinant of hemorheologic improvement was an increase in cardiorespiratory fitness (VO2 max ), correlated with a decrease in plasma viscosity, rather than an improvement in lipid metabolism, although RBC aggregability and deformability exhibited clear relationships with lipid metabolism. For which reason Hct increased in 30% of the patients during this kind of training remains unclear.