We have validated measurements of diameters of the superficial temporal artery and other small arteries in man with a newly developed 20 MHz ultrasound scanner with A, B and M-mode imaging. The diameter of a reference object was 1.202 mm vs. 1.205 mm as measured by stereomicroscopy (nonsignificant). In vitro measurements of porcine carotid arteries could be reproduced with a mean interobserver difference of 0.008 mm, and the repeatability coefficient was 0.04 mm (1.4%). The frontal branch of the human superficial temporal artery (mean 1.24 mm) was measured with intraobserver repeatability coefficients of 0.18 mm (13.8%) to 0.31 mm (23.4%). The interobserver mean difference was 0.01 mm (0.69%) and the interobserver repeatability coefficient was 0.16 mm (11.1%). Pulsatile changes of the cross sectional area of the radial plus the ulnar artery averaged 0.93 mm2 compared to 0.63 mm2 by strain-gauge plethysmography (nonsignificant). Pulsations were 4.6% in the radial artery. We conclude that high frequency ultrasound provides an accurate and reproducible measure of the diameter of small and medium sized human arteries in vivo.