Prognosis of transient ischemic attack (TIA) in 45 patients, diagnosed as TIA out of 75 suspected TIAs, was investigated for an average follow-up of 12.9 months. Twelve (27%) of the 45 patients progressed to complete stroke during the follow-up period. Patients with severe atherosclerotic lesions and with intracranial arterial lesions or with lesions of both intracranial and extracranial sites were predominant among the 29 cases examined angiographically, and the majority progressed to stroke. The prognosis was poorer than in the cases with mild atherosclerotic lesions or cases with only extracranial arterial lesions. From such characteristics of cerebral atherosclerosis of TIA in the Japanese with predominant intracranial severe arterial lesions, a different genesis for the development of cerebral atherosclerosis between Japanese and Americans or other Caucasians may be expected in cases with TIAs. With respect to the relation of recurrence of TIA to prognosis, a single episode of TIA in the past placed the patient at a greater risk of early infarction, where a high incidence (57%) of progression to stroke was found during the follow-up period. Treatment by superficial temporal artery to middle cerebral artery anastomosis or anticoagulant drug medication seemed to improve the prognosis of TIA as compared with antiplatelet aggregation therapy. These results suggest that embolus formation from an embolic source of arterial lesions may play some part in the pathogenesis of TIA and occurrence of subsequent stroke.