Cardiac MRI is a technique that provides information about morphology and function of the cardiovascular system in the form of four-dimensional (4D) scalar data sets. Visualization and extraction of clinically relevant parameters from these data sets may help to diagnose cardiac diseases and malfunctions. Some of these parameters are left (right) ventricle volume, ejection fraction, flow measurements, and wall motion and thickening. Although cardiac MRI is a rapidly growing technique, it must overcome several problems (such as poor spatial resolution, flow and motion artifacts, and low signal-to-noise ratio) in order to produce images with sufficient quality to be used in clinical applications. Existing approaches to visualize cardiac MRI data sets in 4D are based on rendering a geometrical model extracted from the data. In most cases, these models are polygon meshes describing the epicardial and endocardial surfaces of the heart. A wide range of different techniques can be found in the literature to achieve this geometrical model extraction. Our approach consists of applying an iso-surface volume rendering technique in order to visualize the data sets. This visualization includes shape visualization and functional mapping. With this technique, the medical data itself is rendered instead of rendering an extracted geometrical model. This technique has been successfully applied to 3D MRI and CT data sets. Even though the extension of this technique to 4D data sets is not straightforward, the preliminary results are very promising.