Several studies have shown that asymmetric dimethylarginine (ADMA), an endogenous methylated form of the amino acid L-arginine, inhibits NO synthesis and favors oxidative stress and vascular damage. Unlike NO, ADMA concentrations are relatively stable and can be accurately measured in plasma. There is good evidence that higher plasma ADMA concentrations favor atherosclerosis and independently predict adverse cardiovascular and cerebrovascular outcomes in several patient groups. ADMA might represent a unifying pathophysiological pathway linking the presence of vascular risk factors with the onset and progression of cognitive decline and dementia. This review discusses the biological role of ADMA, its potential contribution to the onset and progression of dementia through vascular disease and atherosclerosis, the available evidence linking ADMA with cognitive impairment and dementia, and the strategies to characterize the predictive role of ADMA in cognitive impairment in epidemiological studies. Therapeutic implications and suggestions for future research directions are also discussed.