There is an increased necessity of interest in the recognition of acute myocardial infarction (AMI) occurring during operation or in the early postoperative period. Measurement of serum creatine phosphokinase (SCPK) activity has proven to be useful in the diagnosis and verification of AMI in nonsurgical patients. Fifty-eight patients undergoing thoracic, cardiac, and abdominal operations were studied with preoperative and serial postoperative determinations of SCPK. A marked rise in SCPK activity, related to tissue trauma and injury, occurred postoperatively with the maximal level recorded at 24 to 48 hours after operation, followed by a decline toward normal in five days. Three patients suffered acute myocardial infarctions and had an increased magnitude and duration of SCPK elevation. The results of this study define the pattern of postoperative SCPK activity that normally can be expected, and provide information that may be useful in the recognition of postoperative AMI.