Whole blood filterability was measured in 53 stroke patients (28 patients 40-60 yr and 25 patients greater than 60 yr) to study the hemorheologic parameters, stressing the changes occurring in the elderly. A negative linear correlation was found between whole blood filterability and the hematocrit, plasma viscosity and the serum level of macroproteins (alpha 2-globulin, gamma-globulin and fibrinogen), indicating that whole blood filterability is a collective measure of the different hemorheologic parameters. Whole blood filterability was significantly reduced in stroke patients as compared to normal control values. This reduction could not be attributed to a difference in hematocrit or plasma viscosity, which emphasizes the role of red cell deformability in the filterability of blood of stroke patients. Moreover, whole blood filterability was significantly lower in stroke patients more than 60 yr old than in stroke patients less than 60 yr old. Both plasmatic and red cell factors were contributing factors in the reduction of filterability in the elderly.