BackgroundBreast cancer patients are known to develop brain metastasis at a relatively high frequency. However, imaging findings of brain metastases vary, and it is sometimes very difficult to distinguish these from other tumorous lesions and non-neoplastic lesions, such as cerebral hemorrhage. Meanwhile, there are various causes of cerebral hemorrhage; a major one is cerebral amyloid angiopathy (CAA). With the advancement of imaging technology, CAA-related cerebral hemorrhage can be more precisely diagnosed with magnetic resonance imaging (MRI), but definitive diagnosis of CAA can only be made based on pathological assessment. Herein, we report a case of consciousness disorder appearing during adjuvant therapy for breast cancer. We initially considered that the patient’s cerebral hemorrhage was due to a metastatic tumor, but based on excisional biopsy, she was diagnosed with CAA.Case presentationA 73-year-old Japanese woman underwent curative surgery for left breast cancer. Her disease was hormone receptor-positive and human epidermal growth factor receptor 2 (HER2)-positive invasive ductal carcinoma (pStage IIB). While receiving adjuvant treatment, she developed disorientation, and emergent imaging revealed multiple cerebral hemorrhages. There was no apparent enhancement in the cerebral parenchyma on MRI, and differential diagnosis included hemorrhage due to a metastatic tumor, intravascular large B-cell lymphoma, CAA and thrombotic intracranial bleeding. After hospitalization, the bleeding lesion enlarged, resulting in cerebral hernia, and she needed emergency drainage surgery. The tissue surrounding the hemorrhage was pathologically assessed, and she was diagnosed with CAA. Although we initially suspected the lesion to be a metastatic tumor from breast cancer, there were no tumorous cells.ConclusionAtypical MRI findings made diagnosis difficult in this case, but it should be considered for differential diagnosis when multiple cerebral hemorrhages in elderly patients are observed, especially in cases with symptoms such as transient multifocal neurological deficits and dementia.