['Catheter ablation of typical atrial flutter (AFl) is succesful if double electrograms on the ablation line are widely separated. Nevertheless, a small interval may also be compatible with complete isthmus block. Predicting such a situation may avoid useless additionnal radiofrequency (RF) applications. We postulated that measuring the extra-isthmus activation time (EIAT) on the counterclockwise (CCW) flutter wave is correlated with the extra-isthmus conduction time after a proven block.', 'Files of 76 patients (71 males, 71\u2009±\u200912 years) ablated for typical CCW AFl were reviewed. Ten had 2/1 conduction prohibiting reliable measurement. Three patients with proven crista terminalis shunt were also excluded. In the remaining 63 patients, EIAT was measured on the surface ECG before the first RF pulse from the beginning of the negative deflection of the F wave in lead III to the end of the positive deflection (or beginning of the plateau). After successful ablation and completion of block, right atrial (RA) CCW (during low septal pacing), and clockwise (CW) (during low lateral pacing) activation times were measured.', 'Flutter cycle length was 247\u2009±\u200934 ms and EIAT was 142\u2009±\u200925 ms. A bidirectionnal isthmus block was obtained in all patients after an RF delivery time of 623\u2009±\u2009546 s. At a pacing cycle length of 681\u2009±\u200971 ms, RA CCW and CW activation times were 147\u2009±\u200923 and 139\u2009±\u200926 ms, respectively. There was a good correlation between EIA, RA CCW (r\u2009=\u20090.75, p\u2009<\u20090.0001), and CW (r\u2009=\u20090.69, p\u2009=\u20090.0002) activation times.'] Is new insights into typical atrial flutter ablation : extra-isthmus activation time on the flutter wave predictive of extra-isthmus conduction time after isthmus block?