Minimal invasive surgery can be greatly supported by computer-assisted visualization techniques that provide the surgeon with information which is not obtainable by other means. Real-time direct volume rendering provides interactive high-quality visualization. In contrast to common surface rendering techniques, no information is lost by segmentation, and visualization of small details down to voxel size, as well as the semi-transparent display of arbitrary structures and an interactive modification of all visualization parameters is possible. VIRIM (Visualization In Real tIMe) is the first system that provides these features. It allows the surgeon to interactively explore different approaches to the operation sites for operation planning. During an operation, VIRIM is used to give the surgeon additional real-time information about the endoscope's position in a 3D image and it allows the visualization of objects that are hidden from the endoscope camera. The VIRIM system has be! en fully operational since June 1995 and is being evaluated in the clinic from April 1996 onwards.