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Aims Radiofrequency ablation (RFA) of cavotricuspid isthmus (CTI)-dependent atrial flutter can be performed using various types of ablation catheters. Recent evaluations comparing externally cooledtip RFA (ecRFA) catheters and large-tip (8 mm) catheters found that ecRFA catheter may have a higher efficacy for CTI ablation. The aim of this prospective study was to compare both catheters by stratifying on CTI morphology in order to explain, in part, the discrepancies between previous randomized studies, and to validate predictive factors of difficult CTI ablation on clinical, echocardiographic, and angiographic data. Methods and results Over a period of 24 months, 281 patients were included and stratified on CTI morphology: ‘straight’, ‘concave’, and ‘pouch-like recess’. In straight CTI (n ¼ 150), the duration of application time with a median of 6 min [interquartile range (IQR) 4–9] vs. a median of 12 min (IQR 16–19; P , 0.0001) and the duration of X-ray exposure with a median of 6 min (IQR 4.4–9.7) vs. a median of 10.4 min (IQR 7–17; P , 0.0001) were significantly lower with an 8 mm-tip when compared with ecRFA catheter. In contrast, in concave CTI (n ¼ 95), a trend towards both shorter application time with a median of 12.5 min (IQR 6–23) vs. a median of 19 min (IQR 7–28; P ¼ 0.08) and X-ray duration exposure with a median of 10.4 min (IQR 6–20) vs. a median of 13 min (IQR 8–24; P ¼ 0.08) with an ecRFA catheter when compared with 8 mm-tip catheter were evidenced. No significant difference was shown between 8 mm-tip and ecRFA catheters in the pouch-like recess group (n ¼ 36). Predictive factors of difficult ablation include right CTI length and morphology. Conclusion This study demonstrates that the 8 mm-tip catheter is more effective for ablation in case of a straight angiographic isthmus morphology and that the ecRFA catheter tends to be more effective in case of concave angiographic isthmus morphology. Thus, angiographic isthmus evaluation may predict both the effectiveness of an RF catheter, and the risk of an expensive crossover. These data may explain, in part, the discrepancies of previous studies comparing both catheters.