Moyamoya syndrome has been reported in association with Down syndrome. In paediatric patients, the usual presentation is that of ischaemic stroke. We report a 9-year-old boy with Down syndrome and moyamoya syndrome who presented with acute-onset left hemiparesis. This is the first such reported case in Hong Kong. There is growing evidence that the chromosomal abnormalities in patients with Down syndrome may contribute to a vulnerability for the development of moyamoya syndrome. A high index of suspicion is necessary to make the correct diagnosis. Medical and surgical management strategies for this disease are discussed. Surgical intervention should proceed without delay, if indicated, to prevent further neurological deterioration. A multidisciplinary approach is recommended for the rehabilitation of these patients.