We report an attempt to obtain images of the basal cerebral arteries and to measure the quantitative flow velocity in these arteries from outside the skull via the color Doppler imaging method. In 25 healthy volunteers, 22-41 years old, the transducer was positioned just superior to the zygomatic arch and anterior to the external ear canal. The absolute flow velocity was calculated by dividing the measured velocity by the cosine of the incident angle. The middle cerebral artery was readily identified in all subjects, but the anterior cerebral artery was seen in only 7/50, and the posterior cerebral artery in 27/50. The corrected mean velocity of the horizontal middle cerebral artery was 86 +/- 17 cm/sec at an average incident angle of 49 degrees. However, it was difficult to calculate the absolute flow velocity in the anterior and posterior cerebral arteries because the length of those arteries delineated was too short to read the incident angle. The reproducibility of the mean blood velocity in the middle cerebral artery was tested by two examiners on two different occasions, and showed a linear regression with a correlation coefficient of 0.93 (p less than .001) when the correction with the incident angle was made. Transcranial real-time color-flow Doppler imaging permits more accurate noninvasive quantification of cerebral hemodynamic consequences than previous methods do.