BACKGROUND AND PURPOSE: Moyamoya disease (MMD) is a chronic cerebrovascular disorder with little known etiology. We aim to propose a new classification system for MMD from the perspective of embryology. METHODS: MMD patients' digital subtraction angio-grams were retrospectively analyzed. Every angiogram was analyzed to find the abnormal vessels and from which part of the posterior cerebral artery (PCA) the lesions begin. RESULTS: In 262 MMD cases, 32 pediatric patients had PCA involvement, of which 17 were male and 15 were female; 68 adults had PCA involvement, of which 33 were male and 35 were female. The initially affected part of the PCA was compared between sexes and between pediatric and adult patients, and the findings are not statistically significant (P = 0.233, P = 0.855, P = 0.343, respectively). However, of the 100 cases with PCA involvement, only 4 had the lesions begin from the first part of the PCA, and all of the 4 cases had the basilar artery lesions. All the other 96 cases had the lesions begin from the second part of the PCA or from the posterior communication artery, which is derived from the caudal ramus of the primitive intracarotid artery, leaving the first part of the PCA and basilar artery excluded from affection. CONCLUSION: MMD should be classified into primitive intracarotid artery system-involved type and primitive vertebral basilar artery system-involved type. The reason that the vertebral basilar artery is so rarely involved in MMD might be because of its late development in the brain.