A primary prevention trial had not been possible because an acceptable means of detecting those children at risk of stroke was not available. The advent of TCD to identify arterial abnormalities for the prediction of stroke has provided a means of detection. TCD abnormalities have a high specificity (100 percent) and high sensitivity (90 percent) for detecting angiographically proven narrowing of arterial diameter. Thus, TCD examination of the basal cerebral arteries is predictive of who will develop a stroke.