Many radiological imaging scanners generate high-resolution three-dimensional (3D) images. For complex anatomical regions such as the lungs and heart, these images can be used for interactive navigation inside the anatomy. The 3D image can act as a "virtual environment" representing the anatomy, and a computer-based system can be used for navigating through the environment. But such navigation must occur at interactive speeds if it is to be practical. This demands fast volume rendering from arbitrary viewpoints. The authors present an inexpensive fast volume rendering method that can generate sequences of views at interactive speeds. The method forms part of a system that permits dynamic navigation through 3D radiological images. The authors provide pictorial and numerical results for 3D pulmonary analysis.