Second stereotactic radiosurgery procedures were required in 45 patients with arteriovenous malformations (AVMs) who initially had incomplete obliteration. Repeat radiosurgery was performed at a median of 39 months (range, 24-71 mo) after the first stage. The median AVM volume at the first procedure was 6.0 ml (range, 0.2-18.0 ml). Thirty-seven patients (82%) had AVMs of Spetzler-Martin Grades III through VI. A retrospective analysis revealed definite causes for incomplete obliteration after the first procedure in 33 patients (73%). Incomplete angiographic definition of the nidus was the most frequent factor (57%) associated with failed radiosurgery. Three patients (7%) had recanalization of the AVM nidus after prior embolization; four patients (9%) had incomplete nidus recognition, because AVM vessels were not visualized in the presence of a hematoma. "Radiobiological resistance" was another potential factor associated with failed radiosurgery in 17 patients (38%). Our current technique for volume determination and dose planning includes stereotactic magnetic resonance angiography, magnetic resonance imaging, and complete cerebral angiography (including superselective and external carotid artery injections, as indicated). Integrated multiplanar high-resolution imaging will likely increase the rate of AVM obliteration after stereotactic radiosurgery.