.There is a paucity and inconsistency of data regarding the natural history of patients affected by idiopathic dilated cardiomyopathy (IDCM) and atrial fibrillation (AF). We examined the prognostic implications of AF in a subset of patients with IDCM.We analyzed the data of 539 patients with IDCM enrolled in the Heart Muscle Disease Registry of Trieste.At baseline, 52 (9.6\%) of 539 patients had AF. There was no difference in survival of patients with either AF or sinus rhythm at enrollment (P = .28). During long-term follow-up (90 ± 58 months), AF was detected on ECG/ECG-Holter monitoring in 28 (5.7\%) of 487 patients in sinus rhythm at baseline. Predictors of new onset of AF at multivariate analysis were a more dilated left atrium (OR 1.35, 95\% CI 1.06-1.72; P = .01) and a lower left ventricle ejection fraction (for 10\% decrease, OR 2.41, 95\% CI 1.24-4.69, P = .016). Patients developing AF had higher mortality/heart transplantation rate compared to patients who maintained sinus rhythm during follow-up (P