: We report a case of Moyamoya syndrome developing in association with growth hormone-secreting pituitary adenoma. A 31-year-old female presented with acromegalic features. Magnetic resonance imaging revealed a 1 × 2 cm tumor in the sella turcica and MR angiography demonstrated unremarkable findings. Blood growth hormone and insulin-like growth factor I levels were elevated to 74.1 ng/ml and over 1 575 ng/ml, respectively. The diagnosis was growth hormone-secreting pituitary adenoma, and the tumor was removed through a transsphenoidal approach. Four years after surgery, she visited the outpatient department due to left side weakness for 2 months. Magnetic resonance images showed acute and old infarcted lesions in the basal ganglia and subcortical area and residual small pituitary adenoma in the sellar area. MR angiography demonstrated stenosis of the bilateral distal internal carotid arteries with basal collateral vessels. Conventional cerebral angiography showed complete obstruction in the right internal carotid artery and severe stenosis of the left internal carotid artery, middle cerebral artery, and anterior cerebral artery with basal collateral vessels. Her blood growth hormone and insulin-like growth factor I levels were 15.3 ng/ml and 1 055 ng/ml, respectively. We believe that excess systemic exposures of growth hormone and insulin-like growth factor I may participate in the development of Moyamoya syndrome.