Objective: We investigated the effects of 2-year folic acid and vitamin B12 supplementation on cognitive performance in elderly people with elevated homocysteine (Hcy) levels. Methods: This multicenter, double-blind, randomized, placebo-controlled trial included 2,919 elderly participants (65 years and older) with Hcy levels between 12 and 50μmol/L. Participants received daily either a tablet with 400 mg folic acid and 500 mg vitamin B12 (B-vitamin group) or a placebo tablet. Both tablets contained 15 mg vitaminD3. Data were available for global cognitive functioning assessed by Mini-Mental State Examination (n =2,556), episodic memory (n =2,467), attention and working memory (n 5 759), information processing speed (n=731), and executive function (n =721). Results: Mean age was 74.1 (SD 6.5) years. Hcy concentrations decreased 5.0 (95%confidence interval 25.3 to24.7) mmol/L in the B-vitamin group and 1.3 (21.6 to20.9) μmol/L in the placebo group. Cognitive domain scores did not differ over time between the 2 groups, as determined by analysis of covariance. Mini-Mental State Examination score decreased with 0.1 (20.2 to 0.0) in the B-vitamin group and 0.3 (20.4 to 20.2) in the placebo group (p = 0.05), as determined by an independent t test. Conclusions: Two-year folic acid and vitamin B12 supplementation did not beneficially affect performance on 4 cognitive domains in elderly people with elevated Hcy levels. It may slightly slow the rate of decline of global cognition, but the reported small difference may be attributable to chance. Classification of evidence: This study provides Class I evidence that 2-year supplementation with folic acid and vitamin B12 in hyperhomocysteinemic elderly people does not affect cognitive performance.