<b>Objectives</b><br/> We aimed to determine whether serum levels of homocysteine (Hcy) and its biological determinants, folate and vitamin B₁₂, are related to cognitive decline in elderly people.<br/><b>Methods</b><br/> The concentrations of total Hcy, folate, and vitamin B₁₂ were measured in serum samples from 424 cognitively normal controls, 382 mild cognitive impairment patients, and 56 dementia patients from Ansan Geriatric cohort. The Consortium to Establish a Registry for Alzheimer’s Disease (CERAD) neuropsychological battery was used to evaluate cognitive functions.<br/><b>Results</b><br/> The dementia patients had higher serum Hcy (dementia, 17.6 ± 6.9 μmol/L; control, 12.9 ± 5.0 μmol/L; <i>p</i> &lt; 0.001) and lower serum folate (dementia, 7.9 ± 4.8 ng/mL; control, 10.0 ± 7.1 ng/mL; <i>p</i> = 0.034) levels compared with controls. There was an inverse relationship between Hcy levels and serum folate or vitamin B₁₂ concentrations. The cognitive status as measured by the (CERAD) score was inversely related to Hcy levels. The adjusted odds ratio of dementia was 5.18 (95% confidence interval: 1.91–14.10; <i>p</i> = 0.001) for moderate (30 ≥ Hcy &gt; 15) hyperhomocysteinemia compared with normal Hcy levels (≤15 μmol/L). In addition, there was weak association between low serum folate (&lt;3.0 ng/mL) and the risk for dementia (crude odds ratio = 3.68; 95% confidence interval: 1.07–12.69; <i>p</i> = 0.039).<br/><b>Conclusion</b><br/> Elevated serum Hcy and decreased serum folate concentrations are associated with the risk of dementia in Korean elders.