The effect of recombinant human erythropoietin (r-HuEpo) on whole blood viscosity regarding its relation with hypertension was investigated in this study. 15 patients with mean hemoglobin level of less than 9 gr/dl and without any reason other than uremia for the present anemia were included in the study group. The control group included 10 healthy volunteer with mean hemoglobin level of 12.7 gr/dl. We started r-HuEpo therapy with an initial dose of 50 U/kg three times weekly and increased the dose in order to reach the target hemoglobin level of 10-12 gr/dl. We observed hemoglobin, hemetocrit, whole blood viscosity , and blood pressure changes before and after the therapy. Although the increase in whole blood viscosity was statistically significant (p<0.05) the increase in blood pressure levels were not. According to this result whole blood viscosity was not the only factor playing role in the acceleration or development of hypertension.