Little is known about the accuracy of transcranial Doppler (TCD) sonography in detecting intracranial internal carotid artery (IICA) and middle cerebral artery (MCA) vasospasm. TCD was performed in 49 patients with subarachnoid hemorrhage to evaluate 90 IICAs and 87 MCAs during the vasospasm period. When a mean velocity of at least 90 cm/sec was used to indicate IICA vasospasm, there were 11 positive, 42 negative, 4 false-positive, and 33 false-negative results. Sensitivity was 25% and specificity was 93%. When a mean velocity of at least 120 cm/sec was used to indicate MCA vasospasm, there were 15 positive, 45 negative, 3 false-positive, and 24 false-negative results (15 operator errors). Sensitivity was 38.5% and specificity was 93.7%. When the diagnostic criterion was changed to at least 130 cm/sec, specificities were 100% (IICA) and 96% (MCA) and positive predictive values were 100% (IICA) and 87% (MCA). The authors conclude that TCD accurately detects IICA and MCA vasospasm when flow velocities are at least 130 cm/sec. However, its sensitivity may be underestimated and the importance of operator error, overestimated.