In order to evaluate the optic nerve head perfusion in patients with primary open-angle glaucoma (POAG), we measured the velocity of the red blood cells (RBCs) in the capillaries of the optic nerve head with a laser-Doppler velocimeter and evaluated the blood viscosity by determining the capacity of the RBCs to disaggregate with an erythroaggregameter. Our results showed that in POAG patients optic nerve blood velocity was reduced and that the aggregability of the RBCs was increased. The two parameters were not significantly correlated, possibly because of local papillary autoregulation and anatomical variability in the papilla vessels. These two factors could explain why the same rheological anomaly in two subjects could lead to different responses in blood velocity. The RBC hyperaggregability cannot be explained by quantitative modifications of the plasma proteins. Modifications in the membrane of the RBCs could indeed be responsible for hyperaggregability, since our data suggest that deformability of the RBCs is impaired in glaucoma.