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Atrial fibrillation (AF) contributes significantly to morbidity and mortality in as many as one-third of patients after cardiac surgery that requires cardiopulmonary bypass (CPB). Recent data suggest that inflammatory infiltration of the myocardium may predispose to AF. We conducted an exploratory pilot study to determine if there was an association between the perioperative leukocyte inflammatory response to cardiac surgery/CPB and postoperative AF. We enrolled 72 patients undergoing cardiac surgery with CPB; all patients were in sinus rhythm before surgery. Leukocyte activation (CD11b upregulation) was perioperatively measured in monocytes and neutrophils (PMN). Preoperative C-reactive protein (CRP) and perioperative neutrophil myeloperoxidase (MPO) were also monitored for inflammation, and troponin I was assayed for perioperative cardiac muscle damage. All markers were evaluated for differences between the subset of patients who developed AF versus those who remained in normal sinus rhythm after surgery. All 72 patients completed the study. Postoperative AF developed in 26 (36%) patients. Perioperative monocyte CD11b upregulation was significantly increased in patients who developed AF (P = 0.01), but increases in PMN CD11b were not significantly associated with AF (P = 0.057). The increase in both monocyte and PMN counts after aortic cross-clamp release was significantly associated with postoperative AF (P = 0.007 and P = 0.005, respectively). By contrast, preoperative CRP and perioperative MPO did not differ between AF and normal rhythm patients. Similarly, the peak value of troponin I did not differ between groups. In this pilot study of cardiac surgery/CPB patients, perioperative upregulation of the monocyte adhesion receptor, CD11b, and higher circulating monocyte and PMN numbers were associated with postoperative AF, suggesting that the induction of cellular inflammation during cardiac surgery/CPB may contribute to this pathophysiology.