An atrioventricular pacing thermodilution pulmonary artery catheter was evaluated in 40 patients undergoing cardiac surgery. The catheter was inserted in all study patients in a timely fashion without difficulty and functioned well during the perioperative period. Before the start of cardiopulmonary bypass, atrial capture was achieved in 98% of the patients (threshold mean 4.9 mA), ventricular capture in 100% (threshold mean 3.0 mA), and atrioventricular sequential (AVS) pacing in 98%. After cardiopulmonary bypass, atrial capture was achieved in 95% of the patients (threshold mean 5.2 mA), ventricular pacing in 100% (threshold mean 3.1 mA), and sequential pacing in 95%. In 27 patients in whom cardiac outputs were recorded, cardiac index increased an average of 45% (range 8% to 95%) with atrial or AVS pacing over ventricular pacing alone. In 11 patients, extended atrial pacing was used postoperatively, the longest for 93 hours. No major complications could be attributed to the study catheter. Minor complications included diaphragmatic stimulation in one patient and supraventricular tachycardia, possibly related to atrial pacing postoperatively, in one patient. These data suggest that this catheter / pacing system is effective and reliable for hemodynamic monitoring and temporary atrial or AVS pacing. In addition, the atrial pacing probe can be used perioperatively to record atrial electrograms to facilitate the diagnosis of supraventricular tachyarrhythmias. © 1989 W.B. Saunders Company.