Hemorheological factors were studied in 34 patients affected by retinal vein occlusion (RVO) and in 25 control subjects. Whole blood viscosity, plasma viscosity, fibrinogen and hematocrit were increased in RVO patients compared with controls. Whole-blood filterability was reduced in RVO patients compared with controls. However, when evaluating the type of capillaropathy at fluorescein angiography, blood viscosity and plasma viscosity were altered in both patients with and without evidence of capillary nonperfusion. Reduction of blood filterability showed statistical significance only in patients with capillary nonperfusion. This finding suggests that reduced whole-blood filterability might be a causal factor in the development of capillary nonperfusion in RVO patients.