A 35-year-old man presented with a sudden headache and disturbance of consciousness. On admission, his consciousness level was Japan Coma Scale 100. Computed tomography disclosed a subarachnoid hemorrhage (SAH) and right cerebellar hematoma. Angiography was performed and, at first, arteriovenous malformation of the posterior fossa was diagnosed. Then external decompression of the posterior fossa and ventricular drainage were performed, followed by barbiturate therapy. Repeat angiography revealed that the lesion was a venous angioma with arteriovenous shunts. On day 37, subtotal removal of the lesion was performed. Intraoperatively, acute brain swelling emerged and partial internal decompression of the right cerebellar hemisphere was performed. The postoperative course was comparatively good and the patient was discharged with very mild ataxia. The patient is now being followed up in our outpatient clinic.