BACKGROUND: Atrial fibrillation (AF) is the most common arrhythmic event in patients with hypertrophic cardiomyopathy (HCM). The aim of this study was to identify the clinical impact and prognostic significance of AF on a large cohort of patients with HCM.METHODS: Echocardiographic and clinical correlates, risk factors for AF and thromboembolic stroke and the prognostic significance of AF were evaluated in 509 patients with an established diagnosis of HCM.RESULTS: A total of 119 patients (23.4%) were diagnosed with AF during the index evaluation visit. AF patients had a higher prevalence of stroke and presented with worse functional impairment. Left atrial diameter (LA size) was a common independent predictor of the arrhythmia (OR: 2.2, 95% CI 1.6-3.3) and thromboembolic stroke (OR: 1.6, 95% CI 1.01-2.40). AF was an important risk factor for overall mortality (HR=3.4, 95% CI: 1.7-6.5), HCM-related mortality (HR=3.9, 95% CI: 1.8-8.2) and heart failure-related mortality (HR=6.0, 95% CI: 2.0-17.9), even after adjusting for statistically significant clinical and demographic risk factors. However, AF did not affect the risk for sudden death.CONCLUSIONS: LA size is an independent predictor of both AF and thromboembolic stroke. Moreover, patients with AF, regardless of type, have significantly higher mortality rates than patients without AF.