Moyamoya syndrome (MMS) is a rare, chronic and progressive vasculopathy with a characteristic angiographic pattern and well-recognized predisposing conditions, such as cranial therapeutic radiation. We report the case of a 36-year-old Caucasian female with a history of craniopharyngioma treated with whole-brain radiotherapy 20 years previously. She was admitted to the emergency department with disorientation and imperceptible speech lasting for 1 hour. Upon examination, she had slight motor aphasia, without sensory or motor deficits. However, the neurological deficits worsened on standing position. The computed tomography (CT) angiogram and transcranial Doppler ultrasonography revealed occlusion of the distal portion of the left internal carotid artery (ICA). Mechanical thrombectomy was attempted without success. Head CT was repeated, revealing left periventricular acute ischaemic stroke. The cerebral angiography showed total occlusion of the left ICA with an exuberant network of transdural collateral vessels, confirming MMS. The patient completed a functional rehabilitation program with progressive improvement of deficits and maintained a multidisciplinary follow-up. MMS is a serious late complication from cranial radiation therapy and a well-described risk factor for ischaemic stroke in younger patients. Therefore, early detection and prompt treatment are mandatory, as the consequences can be disastrous, such as cognitive and neurologic decline due to repeated strokes.