To address the problems of surgical risk versus natural risk associated with cerebral arteriovenous malformations (AVM's), and the role of the intravascular operative approach, the authors have assessed a 20-year experience with 450 patients. Results of direct surgery in 90 patients indicate that for the smaller AVM's (Grades I and II), mortality and morbidity rates are lower than a reasonably projected natural risk. Hence, these patients are candidates for surgery in most instances. However, for more extensive AVM's (Grades III and IV), consideration of anticipated future years of exposure to natural risk and the location of the AVM in the brain are necessary for determining operability. In general, neither seizures nor incipient focal neurological dysfunction alone are indications for surgery, and the risks of disability or death from hemorrhage after the fifth decade of life are probably less than the surgical risks by present operative techniques. Considering the usual age of patients at the time of diagnosis, it is estimated that surgical risk is currently less than the natural risk for about 65% to 70% of all AVM patients. The categories of AVM's in which the angiographic effectiveness of the intravascular approach is the greatest correspond to the same categories of AVM's that can be surgically removed with low risk. The intravascular approach is most useful for management of large AVM's causing progressive neurological dysfunction or as a preliminary step to surgery in selected cases in which access to major feeding arteries is difficult. The authors believe that the future of the intravascular approach should be directed toward transforming large inoperable AVM's into operable ones, but that the overall capability for this with acceptable risk is uncertain at present.