OBJECTIVE To determine whether the independently observed increase in the levels of cardiac enzymes in peripheral blood can be accounted for by elevations from postoperative shed-blood reinfusion in patients who have undergone cardiac or thoracic operations. METHODS Prospective, case-controlled study. METHODS A cardiothoracic surgery unit at a university referral centre. METHODS Thirty-four consecutive patients who underwent cardiac or thoracic surgery within a 3-week period. METHODS Coronary artery revascularization (23 patients), cardiac valve replacement or repair (4) and lung resection (7). METHODS Determination of levels of cardiac enzymes and isoenzymes in samples of peripheral and shed blood. Statistical comparison was by paired t-tests within groups and by unpaired t-tests between groups. RESULTS Serum levels of creatine kinase, lactic dehydrogenase and aspartate aminotransferase were significantly (p < 0.001) elevated in samples of shed blood compared with peripheral blood in all groups. The elevations were found to be related to skeletal muscle injury and were not of cardiac origin. In the absence of myocardial infarction, cardiac isoenzyme levels were significantly (p < 0.001) elevated in the peripheral blood of cardiac surgery patients compared with that of thoracic surgery patients. CONCLUSIONS Reinfusion of shed blood will result in elevated levels of cardiac enzymes in peripheral blood but will not increase the percentage of isoenzymes.