Transcranial duplex color-flow sonography provides visualization of intracranial structures, and measures angle-corrected blood flow velocity in the basal cerebral arteries of adults. In 44 patients with central nervous system disease, the oblique diameters of the third and the middle part of the lateral ventricle were measured by transcranial duplex color-flow sonography using a system with a 2.5-MHz transducer, and compared to computed tomography measurements. The correlation coefficients for the third and lateral ventricle measurements were r = 0.83 (p < 0.0001, N = 38) and 0.73 (p < 0.0001, N = 78), respectively. A second investigation was performed by transcranial duplex to evaluate intraobserver and interobserver reproducibility. The correlation coefficient representing interobserver reproducibility in 27 patients was r = 0.87 (p < 0.0001, N = 47) for the lateral ventricle and r = 0.9 (p < 0.0001, N = 49) for the third ventricle. The intraobserver reproducibility correlation coefficient was r = 0.93 (p < 0.0001, N = 22) for the lateral ventricle in 12 patients. In 49 healthy volunteers the oblique diameters of the lateral and third ventricles were age dependent, measuring 16.7 +/- 2.3 mm and 4.8 +/- 1.9 mm, respectively, in those younger than 59 years, compared to 19.0 +/- 2.9 mm and 7.6 +/- 2.1 mm in those 60 years or older. Therefore, transcranial duplex color-flow sonography measures noninvasively the third and the lateral ventricle in adults.