BACKGROUND Heart failure is associated with structural brain abnormalities, including atrophy of multiple brain regions. Previous studies have reported brain atrophy in middle-aged patients with systolic heart failure. In this report, we present the case of a 21-year-old woman with idiopathic dilated cardiomyopathy, cardiac failure, and global cerebral atrophy due to reduced cerebral artery blood flow. We also discuss the impact of brain atrophy in this young adult patient with severe heart failure and no risk factors for atherosclerosis. CASE REPORT A 21-year-old woman with dyspnea and leg edema was admitted to our hospital. After several examinations, an endomyocardial biopsy led to a diagnosis of idiopathic dilated cardiomyopathy, and transthoracic ultrasound cardiography revealed that her left ventricular ejection fraction was 36%. One year after the first hospitalization, her heart failure was classified as New York Heart Association Class III. Magnetic resonance imaging showed severe global brain atrophy, and single-photon emission computed tomography combined with brain computed tomography showed reduced blood flow to the entire brain. She had no risk factors for atherosclerosis and no atherosclerotic changes to her brain or carotid arteries, but her neuropsychological and neurological findings indicated more pronounced brain and cognitive dysfunction. CONCLUSIONS This young adult patient with idiopathic dilated cardiomyopathy, cardiac failure, and global cerebral atrophy showed reduced cerebral artery blood flow and cognitive impairment. The findings of this report indicate that low cardiac output may directly cause brain atrophy in patients with systolic heart failure.