Many factors are involved in the therapeutic intra-arterial management of [human] cerebral arteriovenous malformations (AVM) by embolization. If these factors could be better defined, the suitability of patients for embolization could be more accurately predicted. In 59 embolization procedures, the diameters of the feeding arteries of AVM were analyzed, and a critical diameter found below which emboli are likely to stray into normal vessels and above which embolization can be carried out with a greater chance of success. Neurologic complications occurred on 20 occasions, but most were temporary; in only 1 of 34 patients did a permanent incapacitating complication occur.