OBJECTIVE: Elevated plasma homocysteine has been associated with an increased risk of cardiovascular disease. The association between open-angle glaucoma and cardiovascular disease has recently stimulated interest in the role of homocysteine in the pathogenesis of glaucoma. Some studies report elevated plasma homocysteine in patients with pseudoexfoliation syndrome and pseudoexfoliation glaucoma (PXFG), but have not found this association consistently in other types of open-angle glaucoma. In this study, we have measured plasma homocysteine and C677T methylenetetrahydrofolate reductase (MTHFR) mutation, the commonest genetic cause of elevated plasma homocysteine, in patients with PXFG, primary open-angle glaucoma (POAG), and normal tension glaucoma (NTG). DESIGN: Prospective cross-sectional cohort study. PARTICIPANTS: Forty-eight patients with PXFG, 36 patients with POAG, 34 patients with NTG, and 42 controls without glaucoma. METHODS: Fasting venous blood samples from each participant were analyzed for plasma homocysteine and the C677T MTHFR gene mutation. MAIN OUTCOME OF INTEREST: Mean plasma homocysteine levels, number of individuals with homocysteine above the reference range, and percentage of individuals with heterozygous or homozygous C677T MTHFR gene mutations. RESULTS: Mean plasma homocysteine was significantly higher in PXFG (11.77±4. 18'μmol/L), POAG (11.21±2.84'μmol/L), and NTG (11.74±3. 79'μmol/L) compared with control (9.82±1.75'μmol/L). Hyperhomocysteinemia was found in 27.1% of PXFG patients, 30.6% of POAG patients, and 29.4% of NTG patients. There was no significant difference in frequency of MTHFR C677T gene mutation between groups. CONCLUSIONS: Plasma homocysteine was elevated in PXFG, POAG, and NTG. Elevated plasma homocysteine seems to be associated with glaucoma in these patients.