Objective: Cross-sectional studies have shown that the dysregulation of one-carbon metabolism is associated with cognitive impairment. However, the findings of longitudinal studies investigating this association have been inconsistent. This study investigated the prospective associations between cognitive decline and the levels of folate, vitamin B12 and homocysteine both at baseline and over course of the study period. Methods: A total of 607 (83%) elderly individuals were selected from a group of 732 elderly individuals without dementia at baseline and followed over a 2.4-year study period. The Mini-Mental State Examination (MMSE) was administered to the subjects, and the serum levels of folate, vitamin B12 and homocysteine were assayed both at baseline and at followup examinations. Covariates included demographic data, disability, depression, alcohol consumption, physical activity, vascular risk factors, serum creatinine level, vitamin intake, and apolipoprotein E genotype. Result