Abstract Aims An age of 60 years is often used as cut-off for the diagnosis of idiopathic atrial fibrillation (AF). We investigated the importance of age and atrial size in patients with idiopathic AF and AF patients with isolated hypertension. Methods and results Out of 3978 AF patients in the Euro Heart Survey on AF with known follow-up, 119 (3%) patients had idiopathic AF. We disregarded age and atrial size when selecting idiopathic AF patients since the atria may enlarge by AF itself. For comparison, we selected 152 patients with isolated hypertension from the database. A total of 57 (48%) of the patients were older than 60 years. Persistent or permanent AF was more prevalent in the older idiopathic AF patients (34% in the age <60 vs. 66% in the age >60 years group, P= 0.002) but mean duration of known AF did not differ between these groups [310 days (inter-quartile range, IQR) 60–1827) vs. 430 days (IQR 88–1669), P= 0.824]. Left atrial size did not differ significantly in relation to age (1.50 ± 0.29 mm/kg/m2 in the age <60 vs. 1.56 ± 0.31 mm/kg/m2 in the age >60 years group, P= 0.742). Only two paroxysmal AF patients progressed to permanent AF. No cardiovascular events occurred during the 1-year follow-up. In contrast, strokes occurred in five patients (6%) with isolated hypertension despite similar clinical profile and comparable atrial size as idiopathic AF patients. Conclusion Idiopathic AF may present at advanced age and is even then not associated with significant atrial enlargement, AF progression, or an adverse short-term prognosis. In contrast, elevated blood pressure even when found in the absence of significant atrial remodelling, seems of prognostic importance.