BACKGROUND: Although metabolic syndrome (MetS) has been reported to be associated with cognitive decline and dementia, little was known about late-life MetS and cognitive decline among older old population. The main purpose of this study was to evaluate the role of MetS and cognitive decline among men aged 75 and over in Taiwan. METHODS: This is a prospective cohort study which recruited men aged 75 years and older with intact cognitive function living in the Banciao Veterans Home, a retirement community for veterans in northern Taiwan. All participants received complete history taking, physical examinations, global cognitive tests and laboratory tests. Cognitive status was re-evaluated one year after enrollment to evaluate the role of MetS to cognitive decline in this study population. RESULTS: Overall, 338 people participated in the study and 62 of them were excluded due to low baseline MMSE score, and the remaining 276 people (mean age: 82.4±4.2 years) were enrolled for study. The prevalence of MetS and annual cognitive decline were 22.5% and 15.6%, respectively. During the follow-up period, 9 (3.3%) participants died, 229(83.0%) complete the study. Subjects with cognitive decline were older and had lower serum levels of serum total cholesterol. Multivariate logistic regression showed that older age (OR:1.13, 95% C.I.: 1.01–1.25, P=0.026) and central obesity (OR: 4.19, 95% CI: 1.26–13.91, P=0.019) were independent risk factors for cognitive decline; and MetS defined by Adult Treatment Panel III was a protective factor (OR: 0.20, 95% CI: 0.04–0.94, P=0.041). The protective effect of MetS remained the same when MetS was defined by the criteria of International Diabetes Federation. CONCLUSIONS: Age and central obesity were significant risk factors of cognitive decline, but late-life MetS, however defined, had protective effect on cognitive function. Further investigation is needed to clarify the possible mechanism of MetS and cognitive function in older adults.