A 78-year-old woman was admitted to another hospital with progressive cognitive dysfunction and right hemiparesis. Magnetic resonance imaging (MRI) of the brain revealed a high-intensity area in bilateral frontal lobes on T2-weighted imaging (T2WI) and fluid-attenuated inversion recovery (FLAIR). Brain biopsy revealed no significant findings. She was transferred to our hospital for further examination. Although the etiology of the white matter lesion was not specified, the lesion and symptoms improved with steroid pulse therapy. Three months later, disturbance of consciousness and right hemiparesis worsened again, and the white matter lesion was expanded on MRI. Steroid pulse therapy was again performed, but her condition gradually deteriorated and akinetic mutism developed. T2-based imaging showed microbleeds and analysis of the apolipoprotein E (ApoE) genotype showed ε4/ε4; therefore, cerebral amyloid angiopathy-related inflammation was diagnosed. Two cycles of cyclophosphamide pulse therapy were performed, resulting in improvement of the white matter lesion, disorder of consciousness, and hemiparesis after the first cycle. The patient was discharged from our hospital in an ambulatory state. Steroid therapy for cerebral amyloid angiopathy-related inflammation is relatively well-known in Japan, but cyclophosphamide therapy appears useful for steroid-resistant amyloid angiopathy-related inflammation in the early stage of the disease.