Aims: Atrial fibrillation (AF) is the most common sustained arrhythmia in patients with hypertrophic cardiomyopathy (HCM), and is associated with deterioration in clinical status and outcome. To date, no data have been published pertaining to AF in an Irish HCM population. Methods and Results: 159 patients with HCM attending St Vincent’s University Hospital and Blackrock Clinic, Dublin, were identified. Detailed review of medical notes, Holter monitor, echocardiogram, cardiac MRI (CMR) and implantable cardioverter-defibrillator (ICD) records was performed. Prevalence of AF was 38.4%. HCM patients with AF (HCM-AF) were older(60.6±14.8v54.9±17.3 years, P=0.016) and more symptomatic (NYHA II: 29.7%v16.9%, NYHA III: 4.3%v 1.2%) than HCM patients without AF. History of stroke was recorded in 16.4% of HCM-AF patients, compared with 1% in those without AF. HCM-AF patients had lower left ventricular ejection fraction (echo: 59.5±11.8v68±8, P<0.001; CMR:62.3%v70.5%, P<0.01) and higher left atrial diameter (echo: 49.8±9.5v40.9±7.4, p<0.001; CMR62.3±11.3v70.4±9, p<0.001), compared with those without AF.Myocardial fibrosiswas detected on CMR in 74% of HCM-AF patients and 62% of those without AF. 34% of patients had an ICD in situ, of whom 61% had AF. 24% of these HCM-AF patients received inappropriate shocks, all triggered by AF. Conclusion: AF is common in the Irish HCM population. It is associated with increased risk of stroke, deterioration in symptom status and is a common trigger for inappropriate ICD discharge. We have shown, in-keeping with previous studies, that AF is associated with reduced EF, increased LA diameter and mitral regurgitation in this HCM population.