A number of publications and clinical trials on the management of atrial fibrillation (AF) deal with this arrhythmia as if it represents a single entity. As a result, advances made in recent years have not affected the way AF patients are treated in general practice except, perhaps, for the use of warfarin in anticoagulation. Therefore, there is a need for a classification system and for uniformity in the nomenclature used. The two terms currently used to describe AF, paroxysmal and chronic, require a time frame. It is proposed that if an AF episode lasts longer than 7 days, the condition should be considered chronic. For the first symptomatic, non–self-terminating episode that is fewer than 7 days long, the term recent onset AF may be used, or recent discovery if the AF is asymptomatic or if the duration cannot be determined. Attacks of paroxysmal AF may differ in their duration, frequency, and functional tolerance. In the classification system described, three clinical aspects of paroxysmal AF were isolated in such a way as to have implications for therapy. This classification system was found to be useful for characterizing different subsets of patients with AF.