This paper presents an ultrasound guidance system for needle placement procedures. The system integrates a real-time 3D ultrasound transducer with a 3D localizer and a tracked needle to enable real-time visualization of the needle in ultrasound. The system uses data streaming to transfer real-time ultrasound volumetric images to a separate workstation for visualization. Multi-planar reconstructions of the ultrasound volume are computed at the workstation using the tracking information, allowing for real-time visualization of the needle in ultrasound without aligning the needle with the transducer. The system may simplify the needle placement procedure and potentially reduce the levels of skill and training needed to perform accurate needle placements. The physician can therefore focus on the needle placement procedure without paying extra attention to perfect mid-plane alignment of the needle with the ultrasound image plane. In addition, the physician has real-time visual feedback of the needle and the target, even before the needle enters the patient's skin, allowing the procedure to be easily, safely and accurately planned. The superimposed needle can also greatly improve the sometimes poor visualization of the needle in an ultrasound image (e.g. in between ribs). Since the free-hand needle is not inserted through any fixed needle channel, the physician can enjoy full freedom to select the needle's orientation or position. No cumbersome accessories are attached to the ultrasound transducer, allowing the physician to use his or her previous experience with regular ultrasound transducers. 3D Display of the target in relation to the treatment volume can help verify adequacy of tumor ablation as well.