IMPORTANCE Contemporary guidelines recommend that atrial fibrillation (AF) be classified based on episode duration, with these categories forming the basis of therapeutic recommendations. While pragmatic, these classifications are not based on pathophysiologic processes and may not reflect clinical outcomes. OBJECTIVE To evaluate the association of baseline AF episode duration with post-AF ablation arrhythmia outcomes. DESIGN, SETTING, AND PARTICIPANTS The current study is a secondary analysis of a prospective, parallel-group, multicenter, single-masked randomized clinical trial (the Cryoballoon vs Irrigated Radiofrequency Catheter Ablation: Double Short vs Standard Exposure Duration [CIRCA-DOSE] study), which took place at 8 Canadian centers. Between September 2014 and July 2017, 346 patients older than 18 years with symptomatic AF referred for first catheter ablation were enrolled.All patients received an implantable cardiac monitor at least 30 days before ablation. Data analysis was performed in September 2019.Author affiliations and article information are listed at the end of this article.