In this paper we study the venous drainage of 150 cerebral arteriovenous malformations (AVM), correlating the number of efferent veins with the symptoms of clinical onset and with the size of the lesions. Statistical analysis demonstrates that those AVM's which in the angiograms show only one draining vein have a high risk of haemorrhage. As the number of drainage veins increases, this bleeding risk decreases, becoming quite low in angiomas with multiple drainage veins, which we denominate "medusa-head angiomas". Statistics also demonstrate that small angiomas (less than 2 centimeter on diameter) usually have only one drainage vein, thus carrying a high risk of haemorrhage. As their size increase, so do the number of veins, and the possibility of haemorrhage decreases.