[Background] ECG is insensitive for diagnosing old myocardial infarction. Body surface potential map (BSPM) can provide comprehensive visualization of cardiac electrical activity. Therefore, we investigated (1) how much information BSPM of posterior thorax (P-BSPM) could provide and (2) the validity of P-BSPM in discriminating prior myocardial infarction. [Method] Mattress type harness was designed using 49 capacitively coupled electrodes which demonstrated successful recordings of electrocardiograms from clothed individuals with increased input impedance. P-BSPM of each individual was recorded on the back of the thorax while in clothing. Individuals without any alleged cardiac problems were enrolled as normal control (NC, n=23), RBBB (n=11), or LBBB group (n=3). Patients admitted for follow-up coronary angiography after myocardial infarction were enrolled as old myocardial infarction group (OMI, n=19). Principal component analysis and linear hyperplane approach were used to evaluate the validity of P-BSPM in discriminating prior myocardial infarction from normal control. [Results] (1) In NC, the direction of gradient vector of posterior body surface potential corresponded well with the axis of standard 12-lead ECG. Pearson correlation coefficient was 0.80, 0.96, and 0.85 for P-, R- and T-wave, respectively. When end points of potential gradient vectors were traced during ventricular depolarization, they traveled linearly toward left-inferior field, while those in RBBB showed clockwise rotation and counterclockwise rotation in LBBB. (2) In OMI, initial negativity during QRS was stronger than NC when right coronary artery (RCA) was the culprit (p=0.03). Among the explored BSPM parameters, P-BSPM during ventricular depolarization provided the most powerful discrimination between NC and OMI group (AUC 0.833). P-BSPM was more useful especially for discriminating RCA-territory OMI from NC (AUC 0.981 and 0.705 for RCA and LAD-territory OMI, respectively) than ECG. [Conclusion] With this novel device, P-BSPM was recorded successfully in clothed individuals and provided detailed information for cardiac electrical dynamics. Also P-BSPM was superior to conventional ECG in diagnosing OMI, especially RCA-territory infarction.