Along with the growing technical and functional complexity of modern pacemakers there are mounting difficulties involved in the clinical assessment of unusual episodes of dysrhythmia in pacemaker patients. Very often such episodes go unrecorded except on a simple leectrocardiogram without an extra pacemaker lead. However, explication and diagnosis of the pacemaker patient's electrocardiogram may pose considerable difficulties as natural ectopic depolarization may simulate stimulated impulses, the recording noise may cover up pictures of stimulation impulses etc. The authors describe a computer system developed to provide automatic support to the diagnosis of ECG recorded in pacemaker patients. The system permits the use of any stimulation algorithms including its programmed parameters, as well as evaluation of a manually identified ECG curve which is set as a sequence of "certain" and "potential" depolarization events and stimulation impulses. Computer analysis is introduced to find out whether or not there is logical agreement between the specified pacemaker algorith and the diagnosed ECG, and to reconstruct the heart-pacemaker interaction by means of retrograde modelling of the pacemaker activity. The system generates the actual explanation of the diagnosed episode graphically in the shape of a simulated lay-out lead, complemented with more detailed technical alignment of each step of the pacemaker activity. Three clinical cases are given of the use of the system for analyzing ECG of patients with twin-chamber pacemakers. Limitations of the present system and the need for its further development are likewise discussed.