Prognosis after extracranial-intracranial (EC-IC) bypass surgery has only been studied for a few years and the benefits of this procedure are still controversial. In this single-center retrospective study, we examined the long-term prognosis of patients who underwent EC-IC bypass surgery. Subjects were patients with symptomatic internal carotid artery or intracranial lesion occlusive disease who underwent EC-IC bypass surgery between 1991 and 2003. Of these, long-term prognosis was examined in 57 patients (39 male, 18 female; mean age, 61.8 years) who showed good surgical outcomes 30 days after bypass surgery, measured as a 0-2 on the modified Rankin Scale (mRS). They were divided into 2 groups (survivors and non-survivors) and were analyzed to identify factors effecting long-term survival after bypass surgery. Sixteen patients (28%), whose mean follow-up period (survival time) was 8.3±3.8 years, died after the bypass surgery. The average follow-up period for the survivors was 12.0±1.1 years, which was significantly longer than that for the non-survivors (P<0.0001). At surgery, the non-survivors (mean age 71.5 years) were significantly older than the survivors (P=0.0012). Pneumonia and other respiratory diseases were a frequent cause of death (31.2%), but death by cerebrovascular disease also occurred (12.5%). The rate of recurrent ischemic stroke was 28%, with no significant difference between groups (survivors vs. non-survivors: 31.2% vs. 26.1%, P= 0.82). In the absence of perioperative complications, the long-term prognosis of patients who underwent EC-IC bypass surgery was very good.