Detecting cardiac injury in patients with chest trauma is difficult because the level of the MB isoenzyme of creatine kinase (MBCK) can be elevated from skeletal muscle injury alone. However, the level of cardiac troponin I (cTnl) is not elevated by skeletal muscle injury. To determine whether its measurement would improve the ability to detect cardiac injury in patients with blunt chest trauma, 44 patients were studied. Serial echocardiograms and serial blood samples were obtained. Six patients had evidence of cardiac injury by echocardiography; all had elevations of MBCK and cTnl. One patient had elevations of both MBCK and cTnl with only a pericardial effusion. Twenty-six of the 37 patients without contusion had elevations of MBCK; none had elevations of cTnl. The ratio of MBCK to total creatine kinase improved specificity at the expense of sensitivity. Measurement of cTnl accurately detects cardiac injury in patients with blunt chest trauma and should facilitate the diagnosis and management of such patients.