Advances in digital technology have made the digital processing of X-ray images possible. The principles of processing, which can be traced back to the early origin of radiology, are characterized by the aims "image enhancement" and "functional imaging". These principles have been routinely applied in digital subtraction angiography (DSA) for visualization of non-moving vessels. DSA is used with caution in cardiology mainly because methodical problems caused by the motion of the background structures. In order to assess the clinical relevance of DSA to cardiology the following questions will be answered: Which structures can be visualized? How large is the error of the parameters determined by DSA? What is the clinical relevance of these parameters? What are the advantages of DSA with regard to the examination procedure? Using DSA the left ventricle can be visualized at rest and during exercise by intravenous injection of contrast medium. The examination procedure can easily be combined with measurements of pulmonary arterial pressure. The errors of left ventricular parameters amount to: ventricular volumes: +/- 12- +/- 18 ml; ejection fraction: +/- 8%; ventricular diameters: +/- 7- +/- 13%; wall volume: +/- 48 ml; wall thickness: +/- 1.6 mm. The image quality of aorto-coronary bypass grafts visualized by intravenously injected contrast medium is sufficient to decide whether the grafts are patent or occluded. The visualization of the proximal segments of coronary arteries by intravenous injection of contrast medium is reported in rare cases.(ABSTRACT TRUNCATED AT 250 WORDS)