Analysis of Medicare claims data identified approximately 5.3 million beneficiaries with probable Alzheimer's disease or related dementias that lack formal diagnostic codes in their medical records. This represents 9% of the Medicare population, suggesting a substantial gap between actual dementia prevalence and clinical recognition. The detection gap has profound implications for both individual care trajectories and healthcare system planning. Early identification fundamentally alters disease management, enabling access to emerging treatments like aducanumab and lecanemab that show efficacy only in mild cognitive impairment stages. Undiagnosed patients miss critical opportunities for advance directives, family planning, and lifestyle interventions that may slow progression. From a population health perspective, this hidden burden complicates resource allocation and suggests current screening protocols inadequately capture cognitive decline. The finding aligns with research indicating dementia underdiagnosis rates of 20-50% globally, often due to subtle symptom onset, patient reluctance to seek evaluation, and primary care time constraints. However, Medicare data limitations mean this analysis likely captures moderate to severe cases where functional impairment generates claims patterns, potentially missing earliest-stage disease. This surveillance approach could revolutionize dementia epidemiology if validated against clinical assessments.