The proposed study is a proof-of-concept, randomized, placebo-controlled, double-blind, parallel-group clinical trial assessing the efficacy of 18 months of icosapent ethyl (IPE) therapy on magnetic resonance imaging (MRI), cerebrospinal fluid (CSF), and cognitive biomarkers for AD in 150 cognitively-healthy Veterans ages 50-75 years with increased risk for developing AD due to parental history of the disease and increased prevalence of apolipoprotein E4 (APOE4) allele. The overarching goal of this trial is to assess whether icosapent ethyl beneficially affects intermediate physiological measures associated with onset of AD in order to evaluate whether larger, multi-site, longer-duration Alzheimer's prevention trials are warranted to assess more definitive clinical outcomes. The proposed study aims to: 1) investigate the effects of 18 months of IPE vs. placebo on regional cerebral blood flow as measured by arterial spin-labeling MRI; 2) determine the impact of 18 months of IPE vs. placebo on CSF biomarkers of AD pathology; and 3) evaluate the effects of 18 months of IPE vs. placebo on cognitive performance.