Background Mapping studies in animals have suggested that atrial fibrillation (AF) is based on multiple reentering wavelets. Little information is available about the patterns of activation during AF in humans. The objective of the present study was to reconstruct and classify the patterns of human right atrial (RA) activation during electrically induced AF.Methods and Results AF was induced by rapid atrial pacing in 25 patients with Wolff-Parkinson-White syndrome undergoing surgery for interruption of their accessory pathway(s). The free wall of the RA was mapped using a spoon-shaped electrode containing 244 unipolar electrodes. The activation of the RA during AF showed large interindividual differences. Based on the complexity of atrial activation, three types of AF were defined. In type I (40% of patients), single broad wave fronts propagated uniformly across the RA. Type II (32%) was characterized by one or two nonuniformly conducting wavelets, whereas in type III (28%), activation of the RA was highly fragmented and showed three or more different wavelets that frequently changed their direction of propagation as a result of numerous arcs of functional conduction block. There were significant differences (P<.05) among the three types of AF in median intervals (174+-28, 150+-14, and 136+-16 milliseconds), variation in AF intervals (P sub 5-95) (54+-25, 94+-21, and 104+-22 milliseconds), incidence of electrical inactivity (42+-11%, 21+-4%, and 8+-4%) and reentry (3+-7%, 36+-28%, and 99+-36%), and average conduction velocity during AF (61+-6, 54+-4, and 38+-10 cm/s).Conclusions During pacing-induced AF in humans, the RA is activated by one or multiple wavelets propagating in different directions. Three types of RA activation during AF were identified. From type I to type III, the frequency and irregularity of AF increased, and the incidence of continuous electrical activity and reentry became higher. These various types of AF in humans appear to be characterized by different numbers and dimensions of the intra-atrial reentrant circuits. (Circulation. 1994;89:1665-1680.)