A number of anticoagulants are found in plasma, helping to maintain the balance between thrombosis and haemorrhage. Two of the most important are antithrombin and protein C, which inactivates factors V and VIII. Deficiencies are well-recognised predisposing factors for systemic thrombosis. To establish whether the factor V or Cambridge II antithrombin mutations were present with an increased frequency in patients with idiopathic central retinal vein occlusion (CRVO) we screened 50 such patients. DNA was isolated and the regions of the gene encoding for factor V and antithrombin were amplified by means of the polymerase chain reaction. Following digestion with restriction enzymes the products were electrophoresed in agarose gels. We identified a single patient with the factor V mutation and none with the antithrombin mutation. These findings suggest that resistance to activated protein C and antithrombin mutations does not play a major role in CRVO.