Contralateral acute subdural hematomas that occur during removal of brain tumors under general anesthesia are extremely rare, and there are no reports of this developing during awake craniotomy for brain tumors. We report a case of a 12-year-old boy who complained of sudden and severe headache and nausea around the completion of removal of a glial tumor of the frontal lobe under awake anesthesia. Postoperative computerized tomography scan revealed the presence of contralateral acute minimal subdural hematoma. We suggest that during craniotomy with awake anesthesia for brain tumors, contralateral acute subdural hematoma may occur, even in the absence of brain bulging or changes in vital signs. Sudden intra-operative headache and nausea should be investigated by immediate postoperative computerized tomography scans to ascertain diagnosis.