THE purpose of this study was to determine the difference in prognosis among patients with transient ischemic attacks (TIA) in the carotid arterial system and those with TIA in the vertebral-basilar arterial system. Nearly twice as many patients had TIA in the carotid system as had TIA in the vertebral-basilar system. The survival rates for patients with carotid TIA and those with vertebral-basilar TIA were similar. In both groups, the survival rate was lower than the expected survival rate, but only for the patients with carotid TIA was the difference significant. There was no significant difference in the probability of the occurrence of stroke between patients with carotid TIA and those with vertebral-basilar TIA. These data also indicate that the risk of stroke is much greater soon after the onset of TIA in either arterial system. Among all patients with TIA, the primary cause of death was cardiac disease. The causes of death had a similar distribution for patients with carotid TIA, for those with vertebral-basilar TIA, and for those with "mixed" or "unknown" types of TIA.