Cerebral amyloid angiopathy (CAA) can rarely be complicated by vascular and perivascular inflammation. Patients typically present with subacute cognitive decline, seizures, headaches, and hyperintensities on T2-weighted or fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) sequences. Little is known about the cause and natural history of inflammatory CAA, but recent case series show that some patients respond well to steroids. Here, we describe a patient who presented with severe subacute encephalopathy and was diagnosed with probable CAA-related inflammation based on characteristic MRI, showing subcortical FLAIR hyperintensities with mass effect and multiple cortical microbleeds, after excluding other possible causes of subacute delirium. Mental status examination markedly improved after a course of intravenous methylprednisolone, initially without significant change in MRI appearance. An MRI 4 months later, however, showed marked reduction in the volume of white matter T2 hyperintensities. Mental status remained improved with mild residual retrograde and anterograde amnesia. We propose that the diagnosis of CAA-related inflammation should be considered in patients with profound subacute mental status changes and typical MRI findings. In steroid-responsive patients, clinical improvement can precede radiographic improvement.