For evaluating the spatial location of late potentials (LPs), the authors designed a new system for the body surface mapping of signal-averaged, filtered ECGs using forty-five thoracic unipolar leads (5 x 9 array). The signals from patients with old myocardial infarction (MI, N = 7) and arrhythmogenic right ventricular dysplasia (ARVD, N = 1) were amplified and passed through a bandpass (100-300 Hz) filter. The departure maps, LP isopotential maps, and LP30 area maps were generated and superimposed. The LP30 duration was determined as the section between the filtered QRS endpoints and points thirty milliseconds (ms) before. Isopotential maps of the LPs showed distinct positive and negative regions. In 7 cases with MI, the extreme was related to the zones indicated by the departure maps, and the LP30 area maps also corresponded to the departure areas. In 1 case of ARVD, endocardial fragmented activity directly recorded at the right ventricle closely corresponded with the region on the LP30 area map. In conclusion, body surface LP isopotential maps and LP30 area maps may provide useful information concerning the spatial distribution of endocardial fragmentation.