OBJECTIVE Present criteria for the diagnosis of moyamoya disease emphasize the use of conventional cerebral angiography as an indispensable requirement. However, magnetic resonance angiography (MRA) is fast becoming a reliable diagnostic modality for occlusive cerebrovascular diseases. The purpose of this study is to assess the accuracy of MRA compared with conventional angiography. METHODS Thirty-nine patients (23 children and 16 adults) with moyamoya disease confirmed by conventional angiography (78 side views) were examined with MRA (three-dimensional time-of-flight technique with a 256 x 256 or 256 x 512 matrix). T1- and T2-weighted MR images were also taken. RESULTS The stenotic or occlusive change at the carotid fork was clearly revealed by MRA, which correlated well with conventional angiography (83% good correlation, 17% overestimation). Apparent moyamoya vessels were clearly shown as a signal void on the MR image and fine unusual vessels on MRA, particularly in children with moyamoya disease (82%). However, small moyamoya vessels were poorly shown on both MR images and MRA, particularly in adults (63%). CONCLUSIONS MRA can be an alternative to conventional angiography in typical moyamoya disease cases (the bilateral type of moyamoya disease in children). However, in the early or the end stages of moyamoya disease, diagnosis by means of MRA should be carefully evaluated.