A technique is discussed for producing a robust ultrasound (US)-compatible flow phantom that consists of a thin-walled silicone-elastomer vessel with a lumen of arbitrary geometry, embedded in an agar-based tissue-mimicking material (TMM). The TMM has an acoustic attenuation of 0.56 dB cm(-1) MHz(-1) at 5 MHz, with nearly linear frequency-dependence and acoustic velocity of 1539 +/- 4 m s(-1). The vessel-mimicking material (VMM) has an acoustic attenuation of 3.5 dB cm(-1) MHz(-1) with linear frequency-dependence and an acoustic velocity of 1020 +/- 20 m s(-1). Scattering particles, which are added to the VMM to increase echogenicity and add speckle texture, lead to higher attenuation, depending on particle concentration and frequency. The VMM is stable over time, with a Young's elastic modulus of 1.3 to 1.7 MPa for strains of up to 10%, which mimics human arteries under typical physiological conditions. The phantom is sealed to prevent TMM exposure to air or water, to avoid changes to the acoustic velocity.