Atrial flutter (AFL) and atrial fibrillation (AF) are "fellow-travellers". AF may be a stable, "isolated" rhythm, a bridge between sinus rhythm and AF, or both arrhythmias can coexist. Whether the characteristics of isolated AFL are different from those of patients with AFL combined with AF is still unclear.To compare the clinical characteristics of patients with isolated AFL to those of patients with AFL combined with AF, in a series of patients referred for AFL ablation.Seventy-six consecutive patients (mean age 66.9±12.2 years; 53 men) with a history of electrocardiogram-documented paroxysmal or persistent AFL, referred for catheter ablation, underwent clinical work-up including bidimensional echocardiogram. Patients were subdivided into group I (44 with isolated AFL) and group II (32 with AFL and a history of AF).Underlying heart disease was present in 62 patients (81.6%). Hypertension was the most common cardiac disorder (n=44, 57.9%) and was more prevalent in group II than in group I (75.0% vs 45.5%; P=0.01). Prevalence of prior cardiac surgery was higher in group I (22.7% vs 6.3%; P=0.04). AFL was persistent in 35 group I patients and 17 group II patients (79.5% vs 53.1%; P=0.01). Class I or III antiarrhythmic drug use was more frequent in group II (84.4% vs 45.5%; P=0.001).This study showed significant differences between patients with isolated AFL and those with AFL combined with AF, in the prevalence of underlying heart disease and the use of antiarrhythmic medication, which were higher when both atrial arrhythmias were combined. In turn, the history of cardiac surgery (including atriotomy), was more common in patients with isolated AFL than in those with AFL combined with AF.