Background: Post-pacing interval (PPI) after entrainment pacing is measured from pacing electrodes (PE). Analysis of electrograms recorded in non-pacing, remote electrodes (NPRE) in the differential diagnosis of atrial flutter (AFL) has not been fully understood. Methods: 1. A total of 145 AFLs (84 perimitral or PM AFL, 21 roof-dependent AFL, 13 cavotricuspid isthmus-dependent AFL, 17 focal left atrial tachycardia, and 10 RA flutters) from 95 patients undergoing atrial fibrillation (AF) ablation were included. 2. Multipolar catheters were placed around the lateral right atrium (LRA) and the coronary sinus (CS). E pacing was performed using ablation catheter at pacing cycle length (PCL) 10-30ms shorter than the tachycardia cycle length (TCL). PPIs were measured from CS NPEs after entrainment pacing from the lateral mitral (MA). 3. NPE PPI was defined as the interval from the last entrained beat to the first spontaneous tachycardia electrogram. Results: 1. In peri-mitral AFL (PM-AFL), 81/84 AFLs showed orthodromic activation (OA) of the CS NPRE during E pacing from lateral MA. PPIs from these NPE and from the PE were close (PPI-TCL<20msec) to the TCL. In 3/84 PM-AFL, CS NPREs showed antidromic activation (AA) with PPI shorter than the TCL. 2. In roof-dependent AFL (Roof-AFL), CS NPRE showed OA in 10/21 AFL and AA in 11/21 AFL. Among the 7/11 AA response AFLs, 7 showed PPIs of the proximal CS NPRE shorter than the TCL, suggesting that the proximal CS is part of the reentry circuit, while 3/11 showed AA with PPI longer than TCL. 3. In 13 CTI-dependent AFL (CTI-AFL), CS NPEs were antidromically activated with PPI longer than the TCL. 4. In 14/17 focal LA tachycardia, pacing from lateral MA resulted in OA of CS NPE similar to PM or roof-dependent AFLs. 5. In 10/10 right AT/AFLs, pacing from the lateral MA invariably showed AA of CS NPRE with PPI of PE and CS NPE a lot longer than TCL, suggesting CS NPREs are remote from the reentry circuit. 6. Single E pacing from the lateral MA alone, successfully identified PM-AFL (OA of CS/LRA plus matching PPI at the PE) and CTI-AFL (AA and long PPI at CS NPE plus long PPI at the PE) with a high sensitivity and specificity. Conclusions: Electrogram analysis of the NPRE is useful for rapid localization and differential diagnosis of AFLs during catheter ablation of AF.