Cerebral blood flow (CBF) volume can be measured quantitatively by colour duplex sonography. To test the reliability of CBF volume measurements in newborns, two "blinded" examiners performed a prospective test-retest study in 32 neonates (postmenstrual age 32 to 42 weeks). Measurements were done in the internal carotid and vertebral arteries. Intravascular flow volumes (FV) were calculated as the product of angle-corrected time-averaged flow velocity and the cross-sectional area of the vessel. The CBF volumes measured by the two examiners were very close (mean +/- SD, 62.6 +/- 20.6 vs. 62.1 +/- 21.2 mL/min, NS; coefficient of variation, 6.3%; intraclass correlation coefficient, 0.98). The 95% limits of agreement, according to Bland and Altman, ranged from -7.3 to +8.4 mL/min. In comparison with other test-retest studies, the reproducibility of quantitative CBF measurements reported here is among the best ever found. We conclude that CBF volume can be measured reliably even in preterm neonates.