Transcranial Doppler sonography is commonly used for diagnosis of cerebral vasospasm. However, the overall diagnostic performance of this method in detection of arterial narrowing has not been established. Blood velocity threshold, diagnostic for vasospasm, has been proposed for conventional, "blind" transcranial Doppler sonography (TCD). Nevertheless it cannot be used for transcranial color Doppler sonography (TCCD), because in this method the obtained blood velocities in the major cerebral arteries are higher than are in TCD. The aim of this study was to estimate the accuracy of transcranial color Doppler sonography in detection of middle cerebral artery (MCA) narrowing by means of receiver operating characteristic curve analysis (ROC). One hundred thirty four patients were studied with TCCD immediately before cerebral angiography. There were 75 men and 59 women, age range from 18 to 74 years, mean age 49 years. Of the 268 MCAs examined, 227 arteries were finally included in the construction of the ROC curve. Angiographic vasospasm was graded as none, mild (equal to or less than 25% of vessel caliber reduction--16 patients) and moderate-to-severe (more than 25% of vessel caliber reduction--29 patients). The overall diagnostic performance of transcranial color Doppler sonography in the detection of moderate-to-severe vasospasm of middle cerebral artery was found to be very high. The value of associated area under the ROC curve was 0.94. The value of this area for all vasospasms was 0.85, and this indicates good accuracy of a test. The best performing TCCD parameter for the detection of MCA narrowing was revealed to be peak-systolic velocity. The optimal trade-off between sensitivity and specificity was achieved with a peak systolic velocity of 182 cm/s. Therefore this velocity threshold is proposed as most universal for the diagnosis of vasospasm in the middle cerebral artery.