PURPOSE: To determine the value of magnetic resonance imaging(MR) and magnetic resonance angiography(MRA) inassessing collateral vessels of moyamoya disease. MATERIALS AND METHODS: Twenty-four patients with moyamoyadisease who underwent MR, 3D TOF MRA, and conventional angiography participated in this study. Two radiologistsworking independently and with no knowledge of the angiographic findings, interpreted the MR and MRA images. Todetermine the presence of parenchymal and leptomeningeal collaterals(48 hemispheres) and transdural collaterals(38hemispheres in 19 patients were depicted by angiography of the external carotid), the findings were compared withthose of angiography. RESULTS: Parenchymal, leptomeningeal, and transdural collaterals were depicted byconventional angiography in 34(71%), 32(67%), and 11(29%) hemispheres respectively. The sensitivity andspecificity of MR/MRA for collateral vessels were 79.1/ 88.1% for parenchymal collaterals, 72.1/ 88.1% forleptomeningeal collaterals, and 0.1/18.1% for transdural collaterals, respectively. Respective sensitivity andspecificity of MR/MRA were 88.94/94.1% for leptomeningeal collaterals, and 18.93/55.1% for transdural collaterals,when the prominent posterior cerebral and external carotid artery were regarded as secondary signs ofleptomeningeal and transdural collateral vessels. CONCLUSION: In moyamoya disease, MR and MRA are useful imagingmodalities for the assessment of collateral vessels. The prominent posterior cerebral artery and external carotidartery can be useful secondary signs of leptomeningeal and transdural collateral vessels.