PURPOSE: To report on the occurrence of 5,10-methylenetetrahydrofolate reductase (MTHFR) deficiency, known to cause mild to moderate hyperhomocystinemia and increased risk of vascular occlusive disease, in a young patient with bilateral central retinal vein occlusion. METHODS: A 25- year-old man was initially examined with central retinal vein occlusion in the right eye, followed 4 months later by a central retinal vein occlusion in the left eye. Studies for risk factors predisposing to thrombosis were performed. RESULTS: Hematologic studies failed to detect any pathology. However, the patient was found to be homozygous for 677C-T mutation in MTHFR enzyme. CONCLUSION: Central retinal vein occlusion may be associated with a mutation in MTHFR.