Although radiosurgery is effective in obliterating small arteriovenous malformations (AVMs), it has a lower success rate for thrombosing larger AVMs. The authors surgically resected AVMs from 33 patients ranging in age from 7 to 64 years (mean 30.4 years) 1 to 11 years after radiosurgery. Initial AVM volumes were 0.8 to 117 cm3 (mean 21.6 cm3), and doses ranged from 4.6 to 45 GyE (mean 21.2 GyE). Of 27 AVMs in eloquent or critical areas, 10 were located in language, motor, sensory, or visual cortex, 11 in the basal ganglia/thalamus, one each in the brainstem, hypothalamus, and cerebellum, and three in the corpus callosum. Venous drainage was deep in 13, superficial in 12, or both in eight lesions. Spetzler-Martin grades were II in one, III in 12, IV in 16, and V in four patients. Eight patients experienced rebleeding after radiosurgery but prior to surgery. Three patients developed radiation necrosis and 25 underwent endovascular embolization prior to surgery. At surgery the AVMs were found to be markedly less vascular, partially thrombosed, and more easily resected, compared to those seen in patients who had not undergone radiosurgery. Pathological investigation showed endothelial proliferation with hyaline and calcium in vessel walls. There was partial or complete thrombosis of some AVM vessels and evidence of vessel and brain necrosis in many cases. Complete resection was achieved in 28 patients and partial resection in five. Clinical outcome was excellent or good in 31 cases, and two patients died of rebleeding from residual AVM. Four patients' conditions worsened following microsurgical resection. Final clinical outcome was largely related to the pretreatment grade. Radiosurgery several years prior to open microsurgery may prove to be a useful adjunct in treating unusually large and complex AVMs.