BACKGROUND AND PURPOSE: At diagnosis, the primary clinical manifestations of pediatric Moyamoya disease are TIA or CSs. CSs are reported to be more prevalent in younger than in older children. We sought to determine whether age-related differences in clinical manifestations are associated with age-related angiographic differences. MATERIALS AND METHODS: We divided 78 patients diagnosed with Moyamoya disease before 16 years of age into four 4-year age groups and examined the relationships between age at diagnosis and clinical manifestations and angiographic and MR imaging findings. RESULTS: Among the 4 diagnostic age groups, in those younger than 4 years of age, the prevalence of CSs and of infarctions on MR images was highest, and along with severity of steno-occlusive lesions of the PCA, the prevalence was significantly higher than that in the next diagnostic age group (4-7 years), though the severity of steno-occlusive lesions in the ICA and the degree of transdural collaterals did not differ significantly. The prevalence of CSs and infarctions did not differ significantly in the 3 oldest diagnostic age groups, whereas ICA and PCA lesions and transdural collaterals correlated positively with diagnostic age. CONCLUSIONS: The high prevalence of CSs and infarctions in patients diagnosed before 4 years of age is associated with advanced steno-occlusive lesions of the PCA. In patients 4 years of age and older at diagnosis, transdural collaterals develop in parallel with advancement of ICA and PCA lesions, which may contribute to the nearly constant prevalence of CSs.