An evaluation of the usefulness of estimation of the serum myoglobin in the detection of myocardial necrosis was made in patients with suspected acute ischemic heart disease and in patients in whom elective cardiac catheterization was performed. Measurement of serum myoglobin, by radioimmunoassay, in patients admitted with suspected acute myocardial infarction, suggested that a raised serum myoglobin level was a sensitive indicator of myocardial necrosis. It also showed that the serum myoglobin rose to abnormal levels before the serum creatine kinase. A study of 70 consecutive patients confirmed that the serum myoglobin level is a sensitive indicator of acute myocardial infarction and showed that its sensitivity was greater, and its specificity similar to that of serum creatine kinase. This study allowed calculation of a predictive index for the diagnosis of acute myocardial infarction from the serum myoglobin and serum creatine kinase six hours after the onset of symptoms. The use of a single myoglobin measurement in 114 patients admitted to a coronary care unit was then studied. The proposition that myocardial damage might results from cardiac catheterization was investigated in 115 patients.