The pulmonary veins are the predominant source of ectopic activity initiating AF. The reproducibility of intrapulmonary vein activation during ectopic activity and/or initiation of multiple AF episodes was examined. Eighty-nine pulmonary veins (PVs) among 29 patients undergoing radiofrequency ablation of AF were studied with a 15- to 20-mm diameter, circumferential PV catheter equipped with ten electrodes and a deflectable shaft. Local electrograms were recorded simultaneously during sinus rhythm, ectopic activity, or AF onset, spontaneously or induced via the catheter left in a stable position. Fifty-four arrhythmogenic veins were identified, 39 showing isolated ectopy, and 8 displayed repetitive ectopy (in salvos). The earliest site of activation and the sequence of intra-PV activation during isolated ectopy was identical to that observed during consecutive ectopic complexes in 77% and variable in 23% during isolated ectopy. The earliest activity was sometimes limited to a single bipole. During repetitive ectopy and AF initiation, multiple sources and/or variable activation patterns were noted in 53% of instances, indicating the presence of multiple arrhythmogenic foci within the same PV. Simultaneous electrogram recordings with a circumferential PV catheter identified the presence of multiple arrhythmogenic foci within a single PV.