Three cases of chronic subdural hematoma (CSH) associated with malignancy are reported. Case 1; A one-year-old girl was referred for vomiting and convulsions. Left CSH was removed, and her symptoms disappeared. Cytological examination of chronic subdural hematoma revealed abnormal white blood cells. A clinical diagnosis of acute monocytic leukemia was made after the laboratory examination. Remission was achieved by chemotherapy, but she died one year after the operation. Case 2; A 72-year-old woman was referred for right hemiparesis and urinary incontinence. Left CSH was irrigated, and her clinical symptoms immediately disappeared. Cytological examination of chronic subdural hematoma revealed abnormal white blood cells. A clinical diagnosis of chronic lymphocytic leukemia was made after the laboratory examination. No treatment was given since there were no clinical symptoms of chronic lymphocytic leukemia. Case 3; A 70-year-old woman who had been affected with early gastric cancer and mammary cancer for the previous two years was admitted to our clinic because of headache, right hemiparesis and consciousness disturbance. Left CSH was irrigated, and her clinical symptoms improved. However, there was a tendency to bleed because disseminated intravascular coagulation had occurred, and CT showed bilateral subdural hematoma. A second irrigation was performed, but her symptoms did not improve. Left acute subdural hematoma, which was removed by craniotomy, occurred three days after the second operation. Pathological examination of the outer membrane of the subdural hematoma revealed invasion of adenocarcinoma. She died three days after the third operation. It is recommended that both the cytological and the histological examinations be performed when possible, since they are simple to perform and very useful in some cases.