BACKGROUND: The role of blood viscosity and haemostasis has been investigated in mixed groups of patients with branch and central retinal vein occlusion (CRVO) with conflicting results. This may have partly been due to the different aetiologies of these two types of vein occlusion. METHODS: In this study viscosity and coagulation (including activated protein C resistance) were examined in 87 patients with CRVO and compared with the results from an age-matched, population based control group. RESULTS: Viscosity variables were higher in CRVO than in controls which suggested that reduced red cell deformability was associated with the occurrence of CRVO. A higher percentage of the patients with CRVO (12%) had activated protein C resistance than controls (5%). Patients who developed the complication of iris neovascularisation had relatively low antithrombin III, factor VII, and tissue plasminogen activator indicating both a tendency to thrombus formation and a reduction in fibrinolytic activity. CONCLUSION: Increased blood viscosity may contribute to the production of CRVO by inducing stasis of blood flow, with thrombus formation in at risk individuals who go on to develop iris neovascularisation.