The reduction of blood viscosity is an alternative of the improvement of the cerebral flow during an acute ischemic stroke (AIS). We studied 18 patients with AIS, ranging in age from 44 to 72 years (mean age 57 years), 11 females and 7 males. We applied an isovolemic hemodilution for 2 days starting with an emission of 250 or 500 ml blood, followed by the infusion of an equal amount of 6% HAES-steryl solution. We made determination of hematocrit, plasma density, plasma viscosity 1, 3 and 6 hours before and after the infusion; a decrease was noted in all the studied hemorheologic parameters, and the short term clinical course was an improvement. In conclusion, the isovolemic hemodilution using 6% HAES solution reduced hematocrit, plasma density, plasma viscosity but no changes were noted in the hemodynamic parameters; isovolemic hemodilution as the single therapeutic method in AIS does not solve the therapy, involving only one pathogenetic link, i.e., the microcirculation.