We have examined 120 subjects (52 men, 68 women; age range: 19 to 89) with no vascular pathology. In each subject, we used transcranial color-doppler ultrasonography to measure the calibers and blood-flow velocities of the anterior, middle, and posterior cerebral arteries (ACA, MCA, and PCA, respectively). Generally, the mean measured calibers were lower than those that have been described in cadaveric studies by other investigators. The mean caliber of the MCA was found to be higher than those of the ACA or PCA; this difference was statistically significant (p < 0.05). With age, the calibers of the ACA, PCA and PCA tended to increase, but this trend was not statistically significant. We have noted a statistically significant difference between the left and right sides, the arteries of the left being larger than those of the right. However, there was no statistically significant relationship between vessel caliber and age, sex, or body weight, though the women's arteries tended to be narrower than those of the men. With respect to the maximal and mean blood-flow velocities, no statistically significant relationships with regard to side (left or right), sex, age, or body weight were demonstrable. However, the mean and maximal blood-flow velocities of the MCA were found to be statistically higher than those of the ACA or PCA (p < 0.02). This was especially true under the age of 60 (p < 0.01).