One hundred eleven patients with supratentorial cerebrovascular disease were studied by computerized axial tomography (CT scanning). With one exception, every patient who had a normal scan 48 hours after the onset of symptoms was ultimately diagnosed as having had transient ischemie attack, although in nearly one-third, the clinical diagnosis at the time of the scan was infarction. A normal CT scan, therefore, augurs a good outcome in supratentorial cerebrovascular disease. Ninety-eight percent of the patients with infarction had abnormal scans, with areas of decreased density in a vascular distribution. Pitfalls in the diagnosis of infarction were (1) initially normal CT scans that changed to abnormal after 48 hours, and (2) mass effect of infarction leading to misdiagnosis of brain tumor. Serial studies eliminated both pitfalls. Intracerebral hemorrhages had a distinctive high density appearance. In 43 percent of patients whose scans showed hemorrhage, the clinical diagnosis was thrombosis. Many did not have symptoms, signs, or outcome of cerebral hemorrhage, and the diagnosis would not have been suspect were it not for the CT scan.