Bernard et al. and Chabanel et al. (cited in 1) have shown increased erythrocyte aggregability in patients with retinal venous occlusion. Erythrocyte aggregation plays a major role in determining blood viscosity under conditions of low flow and high vascular resistance, such as are present in the central retinal vein at the lamina cribrosa. Arend et al. (cited in 1) reported increased hematocrit and plasma viscosity as two major determining factors of increased blood viscosity associated with CRVO. McGrath et al., Williamson et al., Trope et al., and Peduzzi et al. (cited in 1) have also reported an increase in blood viscosity, especially for patients with ischemic CRVO. Increased viscosity is also associated with blood dyscrasias (eg, leukemia, polycythemia, thrombocythemia) and dysproteinemias (eg, multiple myeloma, Waldenstrom’s macroglobulinemia, cryoglobulinemia, IgG lambda monoclonal gammopathy, cryofibrinogenemia), including iatrogenically induced hyperviscosity due to intravenous immunoglobulin administration.