PURPOSE: To evaluate the changes in arteriovenous malformation(AVM) of the brain revealed by follow-up MR images after gamma-knife surgery. MATERIALS AND METHODS: In 25 patients with AVM of the brain who had undergone gamma-knife radiosurgery, 39 MR images were obtained during follow-up between 6 and 46 months later, and were reviewed. We evaluated changes volume of the nidus and in its enhancement patterns, changes in T1- and T2-weighted images of the regions in which the nidus had disappeared, and changes in adjacent brain. Conventional follow-up cerebral angiography was performed in seven patients, and the results were correlated with those obtained by M-RI. RESULT: On the MR images obtained as described, disappearance of signal void lesion within the nidus was observed in 16 of the 25 patients. In five of 16, the nidus was obliterated within 1 year of gamma-kinfe radio-surgery, and in the remaining 11 patients this happened after 1 year. In nine cases in which the nidus remained and 16 in which it was obliterated, follow-up examination of MR images revealed spot and mixed spot-nodular enhancement, with signal void lesion. Within the nidus, serial follow-up MR images showed increased enhancement, which was nodule-like. As seen on T1-weighted images, the lesions of obliterated nidus showed slightly lower or iso- signal intensity to gray matter in all 25 cases, while T2-weighted images revealed high signal intensity in 21 cases and iso-signal intensity in four. Abnormal high signal intensity adjacent to the brain was seen on T2 weighted images in nine of 25 cases. In six of seven cases in which cerebral angiography was performed, AVM was obliterated on both MRI and angiography. In one case, however, MR imaging showed that a signal void lesion remained. Cerebral angiography in this case revealed arteriovenous shunting. CONCLUSION: After gamma-knife surgery, early follow-up revealed that in AVM of the brain, a spot and mixed spot and nodular enhancement pattern was visible, with a reduced volume of signal void lesions. During later follow-up, an enhanced nodular pattern was revealed. Nodular enhancement without signal void lesion probably indicates complete regression of the nidus of AVM.