A previously healthy 32-year-old man was surgically treated under a diagnosis of right subcortical hematoma. Magnetic resonance imaging incidentally demonstrated tonsillar herniation. Thirty-two months later, he was readmitted with complaints of occipital, neck, and shoulder pain as well as cerebellar ataxia. Subsequent magnetic resonance imaging demonstrated cerebellar hemorrhage and progression in the downward herniation of the tonsils. Conservative treatment resulted in spontaneous disappearance of the cerebellar hematoma, and the clinical signs and radiological findings improved. Patients with Chiari type I malformation require neuroimaging follow up because the downward herniation of the tonsils can progress in association with subsequent pathophysiological disorders.