To clarify the role of serum CPK-MB assays in the diagnosis of myocardial contusion, we reviewed the hospital records of 182 patients with significant blunt chest trauma and serial CPK-MB determinations. In our laboratory a serum CPK-MB/CPK ratio less than 2.2% is abnormal. The study group was composed of 131 men and 51 women, with a mean age of 35 years. Only six patients had histories of cardiac disease. Of 159 patients injured in vehicular accidents, 143 were drivers, nine were pedestrians, and seven were passengers. There was no relationship between the MB fraction and soft (nonspecific changes) or strongly suggestive (ectopy, focal changes) ECG signs of myocardial contusion. Two of the three patients with power failure had CPK-MB/CPK ratios less than 2.2%. MUGA scans, performed in 18 patients, indicated myocardial injury in seven patients; only 1/7 had abnormal 2D echocardiography, and only 2/7 had elevated MB fractions. In summary, we found no relationship between serum CPK-MB and the presence of clinically significant myocardial contusion. CPK-MB determination in patients with suspected blunt myocardial injury is unjustifiably expensive ($108/assay) and adds confusion to an already vague clinical area.