An apparatus for locating an arrhythmogenic focus or pathway in a heart of a living subject, comprising: a mapping catheter operative for stimulating said heart at multiple locations endocardially or epicardially; and detecting respective sets of pace-mapped electrocardiographic signals at respective locations of said catheter, said catheter having a position sensor; a position processor receiving position signals from said position sensor and operative for determining said respective locations of said catheter; and an analysis processor, adapted for receiving a reference set of electrocardiographic signals and adapted for receiving said sets of pace-mapped electrocardiographic signals from said catheter, said reference and pace-mapped sets of electrocardiographic signals each comprising 12-lead electrocardiograms, said analysis processor operative for correlating said sets of pace-mapped electrocardiographic signals with said reference set of electrocardiographic signals, by calculating a correlation coefficient for a signal from each lead in each set of pace-mapped electrocardiographic signals with a signal from a corresponding lead in the reference set of electrocardiographic signals, making a determination that the correlation coefficients for a predefined number of leads of one of said sets of pace-mapped electrocardiographic signals exceed a predefined value; and responsively to said determination, identifying said arrhythmogenic focus or pathway at said respective location corresponding to said one set of pace-mapped electrocardiographic signals. a mapping catheter operative for stimulating said heart at multiple locations endocardially or epicardially; and detecting respective sets of pace-mapped electrocardiographic signals at respective locations of said catheter, said catheter having a position sensor; a position processor receiving position signals from said position sensor and operative for determining said respective locations of said catheter; and an analysis processor, adapted for receiving a reference set of electrocardiographic signals and adapted for receiving said sets of pace-mapped electrocardiographic signals from said catheter, said reference and pace-mapped sets of electrocardiographic signals each comprising 12-lead electrocardiograms, said analysis processor operative for correlating said sets of pace-mapped electrocardiographic signals with said reference set of electrocardiographic signals, by calculating a correlation coefficient for a signal from each lead in each set of pace-mapped electrocardiographic signals with a signal from a corresponding lead in the reference set of electrocardiographic signals, making a determination that the correlation coefficients for a predefined number of leads of one of said sets of pace-mapped electrocardiographic signals exceed a predefined value; and responsively to said determination, identifying said arrhythmogenic focus or pathway at said respective location corresponding to said one set of pace-mapped electrocardiographic signals. correlating said sets of pace-mapped electrocardiographic signals with said reference set of electrocardiographic signals, by calculating a correlation coefficient for a signal from each lead in each set of pace-mapped electrocardiographic signals with a signal from a corresponding lead in the reference set of electrocardiographic signals, making a determination that the correlation coefficients for a predefined number of leads of one of said sets of pace-mapped electrocardiographic signals exceed a predefined value; and responsively to said determination, identifying said arrhythmogenic focus or pathway at said respective location corresponding to said one set of pace-mapped electrocardiographic signals.