The blood viscosity and related factors were determined in 31 patients with primary open-angle glaucoma (POAG) and 30 normal subjects. It was found that (1) the blood viscosity was significantly higher in the POAG group than in the normal controls, as were also the plasma viscosity, the hematocrit, the fibrinogen content, the index of erythrocyte aggregation and the blood yielding stress, and (2) there was no difference in the value of TK which was a measure of erythrocyte deformability between the two groups. The study showed that patients with glaucoma had blood hyperviscosity which could be caused by the elevated hematocrit and plasma viscosity. The possible relevance of blood hyperviscosity to glaucomatous optic atrophy was discussed.