Vasospasm is the leading cause of death and disability in patients with aneurysmal subarachnoid hemorrhage (SAH). Transcranial Doppler ultrasonography (TCD) can detect the arterial narrowing noninvasively, but the sensitivity and specificity of this technique have not been reported in a population of patients with a high frequency of angiographic vasospasm. In this study, 34 consecutive patients with SAH undergoing angiography during the period of risk for vasospasm had technically adequate TCD examinations within 24 hours of the angiogram. Using a mean flow velocity of 120 cm/sec and above as indicative of vasospasm, TCD correctly detected angiographic vasospasm in 17 patients; there were no false positives. It correctly determined that 5 patients did not have vasospasm, whereas there were 12 false negatives. False negatives were frequently due to angiographic vasospasm involving vessels not assessable by TCD. The correlation between mean flow velocity and the angiographic residual lumen diameter of the middle cerebral artery was statistically significant. These data suggest that TCD is a highly specific (100%), but less sensitive (58.6%) test for the detection of angiographic vasospasm following SAH. Confirmatory angiography may be avoided if the TCD study is positive, but additional studies may be necessary if the clinical picture is suspicious and the TCD study is negative.