We present the MRI findings of six patients with angiographically documented moyamoya disease. All patients underwent CT and MRI, and MRI findings were compared with those of CT. In two patients both preoperative and postoperative MRI were performed. Most of the abnormalities secondary to moyamoya disease were clearly identified on MRI, which proved to be superior to CT, and it was revealed that the effectiveness of surgery as well as the severity of the disease could also be evaluated successfully by this powerful technique.