Two-year double-masked trial of over-the-counter dosage of naproxen sodium vs placebo in 200 cognitively normal participants with a parental or multiplex first-degree family history Alzheimer's disease (AD) dementia. Primary outcomes are decline in cognitive function and slope of change in a summary Alzheimer Progression Score derived from serial assessment of neuroimaging, biochemical, and sensori-neural biomarker indicators of pre-clinical disease -- all believed likely to reflect progress of preclinical AD in this high risk cohort. Approximately 2/3 of participants have volunteered also for serial lumbar punctures for analysis of cerebrospinal fluid. A two-year off-treatment delayed-washout phase is planned to examine sustained treatment effects and evidence of disease modification.