In the Mexican population there is a high prevalence of the C677T MTHFR mutation. The investigators performed this study to evaluate the prevalence of this polymorphism in type 2 diabetic patients with diabetic nephropathy compared with type 2 diabetic patients without nephropathy, besides evaluating the relationship of hyperhomocysteinemia with endothelial dysfunction and microalbuminuria before and after the administration of folic acid. We proposed that the endothelial dysfunction caused by the hyperhomocysteinemia could be reversed after the administration of folic acid.