The purpose of this in vitro study was to assess the effect of 17beta-estradiol on hemorheologic parameters, namely on erythrocyte aggregation and deformability and membrane fluidity. Blood samples from 65 women (aged 57 +/- 4 years) undergoing postmenopausal hormone replacement therapy were obtained and were incubated for 5 min in absence and presence of 17beta-estradiol 10(-5) M. The measured parameters were the erythrocyte aggregation (EAI) and deformability (EI), the acetylcholinesterase activity (AChE), the plasma pH and osmolality and the erythrocyte membrane fluidity assessed by fluorescence polarization with two probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and (1-(4-(trimethylamino)-phenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH). Data analysis was performed using t-Student and Pearson correlation analysis. A statistically significant decrease of the EAI (15.8 +/- 3.02 vs 13.45 +/- 2.3; p<0.001) and an increase of the EI (51.39 +/- 5.64 vs 52.06 +/- 5.36; p<0.01) at shear stress of 30 Pa in presence of 17beta-estradiol 10(-5) M was obtained. There was a decrease in membrane fluidity in 45 blood samples (DPH) and in the other 20 an increase, when beta-estradiol 10(-5) M was present. In vitro beta-estradiol 10(-5) M decreased erythrocyte aggregation in blood samples of postmenopausal women undergoing hormone therapy, which could prevent high blood viscosity and, consequently, cardiovascular events.