BACKGROUND: The diagnosis of moyamoya disease (MMD) is often uncertain. Moyamoya syndrome (MMS) is often misdiagnosed as MMD. High-resolution magnetic resonance imaging (HR-MRI) enables vessel wall assessment to obtain more precise diagnoses. The aim of this study was to determine the true etiologies of arterial steno-occlusion in patients with an angiographic diagnosis of MMD or MMS using HR-MRI. METHODS: HR-MRI was performed in 21 adult patients with angiographically proven MMD or MMS. A definite diagnosis was based on the HR-MRI findings. The diagnoses made via the 2 different imaging technologies were compared, and significant findings were analyzed. RESULTS: A total of 21 patients were enrolled, including 7 patients with angiographically proven MMD and 14 patients with angiographically proven MMS. Among the 7 patients with MMD, HR-MRI confirmed the diagnosis of MMD in 6; the remaining patient was considered to have atherosclerosis in the bilateral distal internal carotid arteries (ICAs) and the left middle cerebral artery. Among the 14 patients with MMS, HR-MRI confirmed MMD in 6 patients (including 2 patients with unilateral MMD), atherosclerosis in 5 patients (including 3 patients with bilateral atherosclerosis and 2 with unilateral atherosclerosis), arterial dissection of the left ICA in 1 patient, and MMD in the left cerebral hemisphere with atherosclerosis in the right hemisphere in 2 patients. CONCLUSIONS: Differentiating MMD from MMS is difficult in certain situations, and HR-MRI may help provide a more in-depth understanding of MMD and MMS, thereby achieving a more reliable diagnosis.