A case of arteriovenous malformation (AVM) with cyst formation is reported. A 14-year-old girl was admitted to our hospital, complaining of headache. On admission, CT scan showed a cystic mass with mural nodule in the right parietal lobe. Contrast enhanced CT scan demonstrated homogeneous enhancement of the nodule. MRI demonstrated the nodule as an area of signal void. Right carotid angiogram showed a typical nidus with feeding artery and draining vein. The nidus was totally removed by the trans-cystic approach. Histological diagnosis was typical AVM with hemosiderin deposits. AVM with cyst formation is very rare, with only six cases having been reported. The mechanism of cyst formation is controversial. Past reports have hypothesized the participation of a massive hemorrhage, or exudative process. Our case showed hemosiderin deposits, but no symptom indicative of hemorrhage. In this case, the etiology of cyst formation was considered to be exudation and thrombosis of the nidus.