This paper presents our experience with intravascular ultrasound imaging of animal and human arteries in vitro and in vivo using a high-frequency (20 M Hz) ultrasound transducer. In vitro, 32 human coronary artery segments were imaged with intravascular ultrasound and compared with corresponding histological sections. Ultrasound and histology measurements correlated significantly (P less than 0.0001) for coronary artery cross-sectional area (r = 0.94), lumen cross-sectional area (r = 0.85) and wall thickness (r = 0.92). In vivo, 19 sheep and eight human common femoral arteries were imaged and the angiographic lumen diameter of 14 animal and six human arteries was compared to the diameter of the corresponding ultrasound images. Significant correlations were found for lumen diameter in animals and humans (P less than 0.001, r = 0.91 and P less than 0.0001, r = 0.96, respectively). These studies demonstrate that this technique can provide high resolution images of arterial vessels and may have unique advantages in diagnosing atherosclerotic vascular disease and in catheter based therapies.