OBJECTIVE Stroke is an occasional devastating complication of cardiac surgery. Transient atrial fibrillation (AF) is a frequent complication of cardiac surgery. Emboli originating from the fibrillating left atrium are a known cause of stroke in the non-surgical setting. The purpose of this quality improvement initiative, conducted by the Wausau Heart Institute, was to characterize strokes after cardiac surgery and to investigate the relationship between AF and the occurrence of postoperative strokes. METHODS We conducted a retrospective record review of all patients undergoing cardiac surgery utilizing cardiopulmonary bypass without associated carotid surgery at our institution between January 1, 1993 and June 30, 1999. The occurrence of strokes and AF was noted. The timing of the AF (duration and relationship to surgery) was recorded. RESULTS Of the 2104 eligible patients, strokes occurred in 68 (3.2%). In 18 patients (27%), stroke was immediately apparent as the patient recovered from anesthesia (intra-operative stroke). Fifty of the 68 strokes (74%) were acquired following the immediate operative period after the patient awoke from anesthesia neurologically intact (postoperative stroke). Postoperative stroke occurred in 2.1% of patients undergoing coronary bypass surgery only, in 2.2% if valve surgery only was performed, and 4.6% if both valve and bypass surgery were performed. AF occurred in 700 patients (33%). The incidence of postoperative stroke was 5.4% in patients with AF and 0.89% in those without AF (P < 0.001). Of those patients suffering a postoperative stroke, 76% had AF following cardiac surgery, compared to 32% if a postoperative stroke did not occur (P < 0.001). Carotid bruits were present in 7 (14%) of the patients with postoperative stroke. Carotid ultrasound studies were performed in 32 patients (63%) and a lesion of > 70% was found in 8 patients (25%). Cerebral lesions contralateral to the stenotic carotid artery occurred in 3 of these 8 patients. CONCLUSIONS Most strokes complicating cardiac surgery occur in patients without significant carotid disease, and are acquired after the patient awakens neurologically intact. The high incidence of postoperative AF in these patients suggests a possible embolic cause for some of the strokes. As such, some postoperative strokes may be preventable.