The usefulness of Clopidogrel as inhibitor of platelet aggregation has been demonstrated, but its effect on hemoreological parameters, such as whole blood viscosity at low and high shear rate, red cell aggregation and deformability indexes, filterability rate and and deformability of red blood cells has not been studied. This study revealed that, in subjects with impaired blood rheology and ultrasound evidence of atherosclerosis, 3-weeks treatment with clopidogrel (75 mg daily) improved blood viscosity at high shear rate and other hemorheological parameters, including red cell filterability rates and dynamic red cell deformability index (p<0.01 for all), without any unwanted side effect. Blood viscosity at low shear rate and red cell aggregation index improved after only 1-week treatment, and the reductions were mantained after two and three weeks (p<0.01). These results indicate that, similarly to another thienopyridine such as ticlopidine, Clopidogrel may have a positive influence on several hemorheological parameters, thus exerting its protection not only through inhibition of platelet function, but also through changes in the hemorheological profile.