A 58-year-old female with gastric cancer presented with left chronic subdural hematoma (CSH) without history of head injury. Magnetic resonance imaging revealed left CSH with atypical findings such as abnormal dural enhancement and swelling of the left cerebral hemisphere. One month after gastrectomy, motor aphasia and right hemiparesis developed. Irrigation of the left CSH was performed. The hematoma was abnormally mucinous and became solid immediately after irrigation. Histological examination showed that adenocarcinoma cells had metastasized to the dura mater and the outer membrane of the hematoma. The preoperative cerebral blood flow (CBF) in the affected cerebral hemisphere, measured by single photon emission computed tomography using N-isopropyl-p-[¹²³I]iodoamphetamine, was much higher than that in the opposite hemisphere, whereas the postoperative CBF was almost equal in both hemispheres. Subdural hematomas secondary to dural metastases of extraneuronal malignancies are rare, and are usually the chronic type. Measurement of the pre- and postoperative CBF in the present patient with CSH following dural metastasis of the malignant tumor showed that preoperative hyperemia in the affected hemisphere may result from dilation of the cerebral vessels caused by the effects of the CSH.