OBJECTIVE Several investigators have tried to assess the role of hyperhomocysteinemia and the 677C-T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene as risk factors in retinal vein occlusion with contrasting results. Aim of the study is to investigate the correlation between increased homocysteine plasma level and the homozygosity for the 677C-T mutation in the gene MTHFR in patients aged under 50 years affected by central retinal vein occlusion (CRVO). METHODS Through a prospective, case-control study, 31 patients under 50 years of age and diagnosed with CRVO were compared with two control groups. The first control group (GROUP I) included 31 subjects matched for age, sex, laboratory tests and the main risk factors for atherosclerosis. The second control group (GROUP II) consisted of 31 volunteers matched only for age and sex. RESULTS The mean homocysteine plasma level was 10.60 micromol/l in patients, 10.39 micromol/l in GROUP I and 9.34 micromol/l in GROUP II. There was no statistically significant difference between mean homocysteine plasma level in cases and in GROUP I. Mean homocysteine plasma level was lower in GROUP II than in patients, and the difference was statistically significant. Homozygosity for the 677C-T mutation in the MTHFR was found in four patients (12.9%), in five controls in GROUP I (16.1%) and in four controls in GROUP II (12.9%). CONCLUSIONS Our results support first of all the hypothesis that the homocysteine plasma level is not a primary and independent risk factor for central retinal vein occlusion, but is more likely a marker of atherosclerosis and the consequence of other well-established risk factors. Second, the importance of the design of the study is highlighted, since the obtained results differed on the basis of the considered control group. This feature could contribute to explain the contradictory results previously reported in the literature.