Background: The Alzheimer’s Disease Cooperative Study-Activities of Daily Living (ADCS-ADL) scale is a versatile functional assessment tool for patients with Alzheimer’s disease (ad). We evaluated its performance in controls, Peruvians with MCI or AD. Methods: A cross-sectional study of older adults attending a neurology institute in Lima (Peru) with mild cognitive impairment (MCI), ad or cognitively healthy. Test-retest reliability (intraclass correlation coefficient, ICC; internal consistency, Cronbach’s alpha) and validity were assessed. Results: We enrolled 276 individuals (ad: 113, MCI: 68, controls: 95) with no age, sex, educational level, and depressive symptom differences. Reliability was ideal (ICC:.996), and Cronbach’s alpha was adequate (.937). The ADCS-ADL could not differentiate MCI from controls but did differentiate ad severity. The ADCS-ADL correlated highly with nearly all tools. Conclusions: The ADCS-ADL scale is reliable in a population with ad in Lima, Peru. Future work may validate a tool for Peruvians with lower educational levels.