The relevance of anatomic and hemodynamic factors to a classification of cerebral arteriovenous malformations. Potential prognostic anatomic and hemodynamic factors were evaluated in 248 patients with cerebral arteriovenous malformations (AVMs), all treated by direct microsurgical removal. The size of each AVM was calculated by its volume, obtained by the multiplication of the three AVM diameters by 0.52. A surgical classification of AVM location (in 11 groups) is proposed. Types of feeders and of drainage were classified as superficial or deep; the extent of the drainage system was classified according to a four-degree scale. The mean flow velocity in the main AVM feeder, detected using transcranial Doppler ultrasonography, was used as an indirect measure of AVM shunt flow in a small number of patients (n = 29). AVM volume was a very important prognostic factor: the incidence of hyperemic complications and the morbidity and mortality rate were significantly higher when the volume of the lesion was greater than 20 cm3 (P less than 0.0001 for hyperemic complications; P less than 0.001 for permanent morbidity and mortality). The incidence of hyperemic complications and the morbidity rate were higher in AVMs in rolandic, inferior limbic, and insular locations than in AVMs in other locations. As for other anatomic factors: a) the presence of deep feeders significantly increased the incidence of hyperemic complications, as well as the morbidity and mortality rate; b) the presence of deep drainage significantly increased permanent morbidity only; c) the extension of the venous system was significantly related to the development of hyperemic complications, and to morbidity and mortality. Transcranial Doppler examination showed that mean flow velocities greater than 120 cm/s in the main feeder were associated with a significantly higher rate of postoperative hematomas and transient deficits. A classification of cerebral AVMs that takes into account AVM volume and location, the type of feeders, the extent of the drainage system, and the main feeder flow velocity is suggested. 