A case of idiopathic adrenal hemorrhage is reported. A 76-year-old woman exhibited a left adrenal tumor, 3 cm in diameter, on abdominal computed tomography. The patient was receiving aspirin medication for atrial fibrillation. There was no evidence of increased adrenal hormones. The mass enlarged to 6 cm in diameter within 18 months, and malignancy was suspected. The mass was diagnosed as adrenal hematoma by operative findings.