SUMMARY Fifty consecutive patients, 25 undergoing aortic valve replacement and 25 mitral valve replacement, were studied by serial electrocardiography, preoperative and postoperative technetium-99m pyrophosphate radionuclide scanning, and serial measurement of enzymes (creatine kinase, aspartate aminotransferase, urea stable lactic dehydrogenase) and the MB isoenzyme of creatine kinase to define the incidence of peroperative myocardial infarction and to identify the most appropriate diagnostic techniques. The use of myocardial scanning and measurement of peak enzyme activity proved to be accurate indicators of myocardial infarction, but the electrocardiogram was of limited value. The measurement of creatine kinase MB isoenzyme had no diagnostic advantage over that of the other enzymes. There were two deaths in the series, one due to acute pancreatitis after aortic valve replacement and the other due to myocardial injury after mitral valve replacement. There were four non-fatal myocardial infarctions after aortic valve replacement, giving an incidence of 16%, and none after mitral valve replacement, giving an incidence of 4%.Peroperative myocardial infarction is the major cause of mortality and morbidity after cardiac valve replacement. 1 2 There continues to be a great research effort into improving methods of protecting the heart from such injury sustained during the period of global myocardial ischaemia when the aorta is cross clamped during replacement of the valve.3-5 The importance of the length of the aortic clamping time has been stressed by Stiles and Kirklin. A "safe" time limit for aortic cross clamping will vary according to operative factors such as the method of myocardial protection used and the preoperative clinical and haemodynamic status of the patient.5 No single best method of myocardial protection is agreed, and, although the operative mortality for aortic valve replacement has declined, there remains a high incidence of operative myocardial injury. Varying estimates of the incidence of between 4% and 26% have been reported after aortic valve replacement,68 and of between 00/o and 13%