This study was undertaken in 79 patients with retinal vein occlusion to assess the different systemic mechanisms contributing to the occlusion, namely, intrinsic vessel disease and abnormalities of the blood constituents and blood viscosity. In 55 patients older than 50 years of age, important associations were hypertension, abnormal results on glucose tolerance test, hyperlipidemia, chronic lung disease, and elevated serum IgA levels. In the 24 patients younger than 50 years of age, male incidence was high and important associations were head injuries, hyperlipidemia, and the use of estrogen-containing preparations. Hyperviscosity and cryofibrinogenemia were prominent in both groups. The pathogenesis of retinal venous occlusion is complex involving interaction between the vessel wall and blood constituents.