To evaluate electrophysiological changes after bilateral pulmonary vein isolation in patients undergoing mitral valve surgery and to relate the clinical outcome at 1-year follow-up with the results of the postoperative electrophysiological study. Prospective study of patients with atrial fibrillation undergoing bilateral pulmonary vein isolation using radiofrequency energy and concomitant mitral valve surgery. Pulmonary vein isolation was performed with a heptapolar catheter. Each set of up to 7 simultaneous applications aimed at a maximal duration of one minute and a maximal preset temperature of 70 degrees C. Energy delivery (< 150 watts) was controlled by thermosensors located on each electrode. Before hospital discharge, all patients presenting sinus rhythm underwent an electrophysiological study, using epicardial pacing wires placed during surgery. Isolated areas were compared to the non-isolated left atrium with respect to electrogram amplitudes and atrial capture thresholds. At one-year follow up, cardiac rhythm was assessed in all patients and correlated with the results of the postoperative electrophysioloical study. The study population consisted of 20 patients with mitral valve disease (mean age 59 +/- 11 years), chronic atrial fibrillation present for 0.5 to 18 years and dilated left atria (55 +/- 11 mm assessed by M-mode echocardiography). The radiofrequency procedure added, on average, 7 minutes (3 to 17) to the duration of the mitral valve surgery. No patient died or suffered surgical complications. Eleven patients presented sinus rhythm before hospital discharge. The electrogram amplitude inside the isolated zones was less than 25% of that in nonisolated areas (0.3 +/- 0.2 mV versus 2.1 +/- 1.7 mV, p = 0.002). Atrial capture thresholds were significantly higher for the isolated areas (13.5 +/- 9.3 mA versus 8.5 +/- 4.0 mA; p < 0.05). At one-year follow-up, nine patients maintained atrial fibrillation, nine remained in sinus rhythm and two lost sinus rhythm to atrial fibrillation. In both these patients, the reduction in electrogram amplitude inside the isolated zones was similar to that of the remaining patients discharged in sinus rhythm. However, postoperative electrophysiological studies in these patients showed that atrial capture thresholds for isolated areas had not increased significantly (8 mA and 6 mA, respectively). Rhythm at discharge after bilateral pulmonary vein isolation tends to remain at one-year follow up. Radiofrequency-induced modifications in electrogram amplitudes may be less important than increases in pacing thresholds of isolated zones with respect to maintenance of sinus rhythm.