Introduction: Arrhythmias are common in patients with idiopathic pulmonary arterial hypertension (IPAH). However prospective analysis of arrhythmias in a large cohort of IPAH patients using implantable cardiac monitors (ICMs) is lacking. Aims: Establish the rate of arrhythmias in a prospective cohort of IPAH patients using ICMs. Determine factors that predict arrhythmia occurrence in IPAH. Elucidate whether arrhythmia incidence is related to adverse outcomes. Methods: ICMs were implanted into 80 IPAH patients across two UK centres. Arrhythmia and clinical worsening events (defined as death, transplant or hospitalisation due to PAH worsening) were prospectively recorded and characteristics associated with arrhythmia incidence identified. The arrhythmia rate in IPAH patients was compared to 72 age and sex-matched patients without IPAH who had ICMs implanted for clinical indications. Comparisons of data were performed using analysis of variance, χ2 or Kruskal-Wallis calculations as appropriate. The Cox proportional hazards model was used to resolve independent risk factors for significant arrhythmia occurrence. A two-tailed probability level of <0.05 was considered significant. Results: Over 186 patient-years follow-up 79 arrhythmia events were noted in 40% IPAH patients vs 78 events in 41% comparators. Arrhythmia incidence was related to right atrial (RA) enlargement at time of implant, baseline conduction disease and symptom reporting. Arrhythmia incidence predicted clinical worsening (figure).20% of patients had targeted arrhythmia intervention as a direct consequence of ILR studies. Only RA size was independently associated with clinically significant arrhythmia on multivariate analysis Conclusion: Arrhythmias are frequent in IPAH patients and predict worse outcomes. RA enlargement is independently associated with clinically significant arrhythmia incidence. IPAH should be considered a high-risk group for continuous monitoring studies.