OBJECTIVE: Moyamoya disease (MMD) is a cerebrovascular disorder, currently defined as progressive stenosis of intracranial internal carotid artery and its main branches with secondary formation of netlike vessels. Its precise formation mechanism, however, is unknown. We propose that MMD is caused by abnormal embryonic development of the cerebral arteries and also propose a mechanism for MMD formation. METHODS: The anterior, middle, and posterior cerebral arteries, anterior choroidal arteries (AChoA) and posterior choroidal arteries, and posterior corpus callosum arteries were analyzed separately for each patient with MMD to determine whether the arteries exhibited the following characteristics, which we regarded as remnants of primitive vessels: 1) plexiform arteries rather than normal artery trunks; 2) clustered arteries converging at locations of normal artery trunks; and 3) dilated AChoA. RESULTS: We retrospectively analyzed 39 consecutive patients with MMD, of whom 30 had anterior cerebral arteries, 31 middle cerebral arteries, 10 posterior cerebral arteries, 30 AChoAs, 18 posterior choroidal arteries, and 20 posterior corpus callosum arteries and had the characteristics of primitive vessel remnants. Altogether, 82.05% of the patients had the remnants of primitive vessels. CONCLUSIONS: MMD is more likely to be caused by abnormalities of cerebral artery development, which mainly occur in the embryonic period or postnatally. The abnormality developing processes include sprouting angiogenesis, vessel fusion, and pruning, which primarily affect cranial ramus of primitive internal carotid artery, may occasionally affect the caudal ramus of the primitive internal carotid artery, and rarely affect the vertebrobasilar artery system. So-called moyamoya vessels comprise un-fused primitive small vessels; to compensate, enlarged AChoAs remain undegenerated but are not dilated.