We developed a system for augmented reality visualization of ultrasound images. This system is based on our earlier "augmented workspace" setup. The user wears a custom video-see-through head-mounted display (HMD). Two color video cameras attached to the HMD provide a stereo view of the scene. A third head-mounted video camera is added for tracking. A set of markers is attached to the ultrasound transducer; a set of stationary markers can be positioned above the workspace. The system runs at the full 30Hz video frame rate with a latency of about 0.1 sec, generating a stable augmentation with no apparent jitter visible in the composite images. Three SGI Visual Workstations provide the computing power for the system. In this paper we describe the details of the system, its calibration with a configuration of optical markers partially immersed in a water bath, and some system features like spatiotemporal freezing and 3D target localization that promise to be helpful for practical applications.