We recently developed a wideband 15-MHz linear array probe (15 M) with a band width of 8 MHz (9-17 MHz). Both axial and lateral resolution of 15 M, evaluated using a phantom model, were better than those of the current 10-MHz linear probe. To compare interobserver variability in measurement of medium-sized muscular arteries acquired using a 7.5-MHz linear probe (7.5M), a 10-MHz linear probe (10 M) and 15 M, two observers independently acquired images of the brachial and radial arteries, and measured the diameter and intima-media thickness (IMT) of those arteries in 17 male volunteers. Intraobserver variability in determining percent flow-mediated dilatation (%FMD) was assessed in the same subjects using 15 M. Coefficients of variation (CV) in arteries measured using 7.5 M, 10 M and 15 M were 7.0%, 2.5%, and 1.5%, respectively, for the diameter of the brachial artery; 10.3%, 5.8%, and 3.2%, respectively, for the diameter of the radial artery; and 17.0%, 13.8%, and 8.5%, respectively, for IMT of the far wall of the brachial artery. The CV of measurement of %FMD was 4.6%. The new 15-MHz probe thus warrants use in evaluating morphology and function of muscular arteries of medium size.