Moyamoya disease is characterized by bilateral involvement of the internal carotid arteries. The etiology of this involvement is unknown. However, we previously reported two pediatric cases of moyamoya disease that progressed from unilateral to bilateral involvement. Some cases of unilateral occlusion in the carotid fork seem to have occurred at an early stage of definite moyamoya disease. In the present study, we examined five pediatric patients showing bilateral and/or unilateral occlusion of the internal carotid artery. In each case, the etiology was known. They included Apert syndrome, radiation therapy for pilocytic astrocytoma, systemic lupus erythematosis, von Recklinghausen disease and Down syndrome. The clinical manifestations, EEG, CT, MRI, PET, and angiographic findings in these patients were presented in comparison with those of definite moyamoya disease. This comparison led us to suggest that definite moyamoya disease might be included in these cases, and we emphasize the importance of precise examination in each case.