Abstract: The visualization of iso-surfaces from gridded volume data is an important tool in many scientific applications. Today, it is possible to ray trace high-quality iso-surfaces at interactive frame rates even on commodity PCs. However, current algorithms fail if the data set exceeds a certain size either because they are not designed for outof- core data sets or the loading times are too high because there is too much overhead involved in the out-of-core (OOC) techniques. We propose a kD-tree based OOC data structure that allows to ray trace iso-surfaces of large volumetric data sets of many giga bytes at interactive frame rates on a single PC. A LOD technique is used to bridge loading times of data that is fetched asynchronously in the background. Using this framework we are able to ray trace iso-surfaces between 2 and 4 fps on a single dual-core Opteron PC at 640×480 resolution and an in-core memory footprint that is only a fraction of the entire data size.