Identical twin cases of moyamoya disease associated with persistent primitive trigeminal artery variant are presented. Both of the children suffered from the cerebrovascular occlusive disease called "moyamoya disease," but there existed a remarkable time lag in the manifestation of their first clinical symptoms. Coexistence of persistent primitive trigeminal artery variant and high occurrence of moyamoya disease in identical twins suggest some congenital factors in the development of this disease. However, the time lag of the first clinical manifestation between the twins suggests certain acquired factors may also play a role in the manifestation of this disease.