Two years after being successfully treated for biopsy confirmed primary angiitis of the central nervous system (PACNS), a 69-year-old woman presented with cognitive decline. In contrast to her first presentation, her altered mental function developed gradually, was not associated with headache or abnormal cerebrospinal fluid analysis, and did not improve with immunosuppression. Reevaluation of her original brain biopsy not only confirmed the presence of PACNS, but also revealed neuritic plaques and neurofibrillary tangles, suggesting a concurrent diagnosis of Alzheimer's disease. Cerebral angiogram did not suggest vasculitis and magnetic resonance imaging showed generalized cerebral atrophy supporting the diagnosis of Alzheimer's. This case illustrates that Alzheimer's dementia and PACNS can coexist in a single patient and that Alzheimer's disease should be considered when a patient with successfully treated PACNS presents with cognitive decline months or years after initial diagnosis.