Background: Cardiac tamponade is a severe complication after open heart surgery. Diagnostic imaging is challenging in postoperative patients, especially if tamponade develops with subacute symptoms. Hypothesizing that delayed tamponade after open heart surgery is not sufficiently detected by transthoracic echocardiography, in this study CT scans were used as standard reference and were compared with transthoracic echocardiography imaging in patients with suspected cardiac tamponade. Method: Twenty-five patients after open heart surgery were enrolled in this analysis. In case of suspected cardiac tamponade patients underwent both echocardiography and CT imaging. Using CT as standard of reference sensitivity, specificity, positive and negative predictive values of ultrasound imaging in detecting pericardial effusion/hematoma were analyzed. Clinical appearance of tamponade, need for re-intervention as well as patient outcome were monitored. Results: In 12 cases (44%) tamponade necessitated surgical re-intervention. Most common symptoms were deterioration of hemodynamic status and dyspnea. Sensitivity, specificity, positive and negative predictive values of echocardiography were 75%, 64%, 75%, and 64% for detecting pericardial effusion, and 33%, 83%, 50, and 71% for pericardial hematoma, respectively. In-hospital mortality of the re-intervention group was 50%. Conclusion: Diagnostic accuracy of transthoracic echocardiography is limited in patients after open heart surgery. Suplemental CT imaging provides rapid diagnostic reliability in patients with delayed cardiac tamponade.