Ninety two 50-year-old-or-more patients with acute (less than 72 hours) cerebral infarction involved middle cerebral artery region and hematocrit of 40% or more were prospectively randomised to either hemodilution group (by rapid venesection, venous infusion of autologous plasma and 500 ml dextran 40) or standard therapy group. Effects of hemodilution on acute cerebral infarction were evaluated by clinically neurological deficit scores, hemorheological parameters and size of infarction. The results of our clinical trial are: 1. Clinical efficacy of hemodiluted patients is significantly superior to that of standard treatment patients. Effective rates are 63.04% and 41.30% respectively (P less than 0.05). 2. At 48th hour after hemodilution there are profound decreases in hematocrit, blood viscosity at high shear rate and blood viscosity at low shear rate. These changes last more than four weeks. Whereas in standard group, hemorheological parameters do not evidently change. 3. Sizes of cerebral infarction do not distinctly change in both groups between at entry and at the fourth weekend after treatment.