The clinical features of 6 children who are probable sufferers of "unilateral" Moyamoya disease are here reported. They showed angiographic findings which were compatible with those of Moyamoya disease, albeit only on one side. They did not, however, show any basic aetiologic factors. The age of onset, the clinical symptoms and the findings of electroencephalography, angiography, and positron emission tomography in these cases were also quite similar to those in the cases of Moyamoya disease except for unilateral involvement. All 6 patients underwent either direct or indirect EC-IC bypass surgery. In 3 children who received encephalo-duro-arterio-synangiosis, an indirect bypass procedure, the collateral circulation was well formed postoperatively. In the follow-up study, 2 of the 6 cases starting with a unilateral lesion developed bilateral involvement later. However, the other 4 cases persisted in showing only unilateral involvement. These 4 cases may suggest the existence of "unilateral" Moyamoya disease in the paediatric age, and it is recommended that such cases be treated similarly to those of bilateral Moyamoya disease.