Metformin may affect the risk of atherothrombotic disease. However, metformin increases levels of homocysteine (Hcy), considered an independent risk factor for atherosclerosis. We evaluate whether homocysteine-lowering has a beneficial effect on arterial elasticity and metabolic parameters in metformin-treated diabetic patients. In double-blind, placebo-controlled study, 60 diabetic patients treated with high dose of metformin were randomly assigned to receive daily oral supplementation with folate (1000 mcg), vitamins B12 (400 mcg) and B6 (10mg) (group 1) or placebo (group 2). Lipid profile, HbA1C, insulin, C-peptide, hs-CRP, vitamin B12, folic acid, homocysteine, endothelin, homeostasis model assessment-insulin resistance (HOMA-IR) were measured. Arterial elasticity was evaluated using pulse wave contour analysis (HDI CR 2000, Eagan, MN). The two groups were similar at baseline in terms of hemodynamic and arterial elasticity parameters. After a 4-month small artery elasticity index (SAEI) was significantly greater in patients who received Hcy-lowering agents than in the placebo group: 4.3+/-2.04 ml/mm Hg x 100 versus 3.2+/-1.1 ml/mm Hg x 100, p=0.01. Post-treatment vitamin B12 and folic acid levels were greater in group 1 versus group 2: 738.1+/-279.9 pg/ml versus 566.1+/-167.4 pg/ml, p=0.007 and 14.9+/-4.8 ng/ml versus 8.3+/-2.9 ng/ml, p<0.0001, respectively. Hcy level decreased significantly in the treatment group from 10.0+/-4.4 to 7.6+/-2.5 micromol/l, p=0.002 and did not change in placebo group (p=0.9). Hcy-lowering therapy improved small arterial elasticity in diabetic patients treated with high dose of metformin. The improvement was associated with a decrease in Hcy as well as an increase in folic acid and vitamin B12. These findings suggest that Hcy-lowering may have beneficial vascular effect in metformin-treated diabetic patients.