Current biomarkers, including cerebrospinal fluid (CSF) and positron emission tomography (PET) assessments for amyloid and tau, often fail to predict cognitive decline in a substantial proportion of LOEU patients. This case report presents a 67-year-old man with LOEU who later developed dementia with Lewy bodies (DLB). The gene discussed is MAPT; the disease is Lewy body dementia.