Introduction: Atrial fibrillation (AF) and atrial flutter (AFL) are frequently associated. The aim of the study was to look for the prevalence of previous history of AF in patients referred for radiofrequency ablation of AFL and the possible causes. Methods: 1061 patients, 819 males, 242 females, mean age 64±12 years were referred for ablation of recurrent or poorly-tolerated typical AFL. Clinical history, previous AF, data of echocardiography, was collected. Patients were followed from 3 months to 10 years (mean 3±3 years). Antiarrhythmic drugs (AAD) were stopped after ablation except in patients with previous AF before ablation. Results: 336 patients had presented at least one episode of sustained AF (>1 min) before AFL ablation (group I). The prevalence of AF in this population was high (32%). These patients were compared to remaining patients without know AF (group II). Ages of group I and II were similar (65±11 vs 64±12 years). They were more frequently females in group I (27%) than in group II (24%) (p<0.04). The presence of associated heart disease (HD) was less frequent in group I (67.5%) than in group II (89%) (p< 0.0001).When the nature of HD was analysed, the frequency of dilated cardiomyopathy (10 vs 10%), hypertensive disease (22 vs 23%), valvular HD (10 vs 14%) did not differ. Group I had less respiratory failure (4 vs 13%) (p<0.0001), congenital HD (2 vs 5.6%) (p<0.005), and ischemic HD (14 vs 21%) (p<0.004) than group II. Diabetes and previous strokes were as frequent in group I (10, 4%) as in group II (14, 3%). Presentation with rhythmic cardiomyopathy or 1/1AFL was as frequent in group I (7, 11%) as in group II (9.5, 8%). Most patients of group I received an AAD (AA) (87 vs 66%) (p<0.0001), mainly a class I AAD (45 vs 9%) (p<0.0001). After AFL ablation, AF occurrence was more frequent in group I despite AAD maintaining (31%) than in group II (22%) (p<0.0044). Treatments of AF as ablation of His bundle and pacemaker or ablation of AF were more frequently used in group I (3.5, 5%) than in group II (0.8%,p<0.0001, 0.3%,p<0.0011). However cardiac mortality was similar in both groups (5 vs 8%). Conclusions: There is an high prevalence of history of AF in patients referred for ablation of AFL (32%). Paradoxically, age and presence of HD were not favoring causes. Females without HD were at higher risk to have both arrhythmias than males. Their risk of subsequent AF was higher than in patients without previous AF, but cardiac mortality was similar.