Background: Acute pericarditis is a close clinical mimic of pulmonary embolism (PE) in the emergency department, and thus many of these patients are evaluated with chest computed tomography (CT). Purpose: To study whether CT findings can be diagnostic of acute pericarditis. Material and Methods: Using the electronic medical record, we retrospectively identified 46 cases of acute pericarditis and 46 control patients with pericardial effusions due to volume overload, all of whom underwent CT examination. Cases were reviewed by two blinded academic thoracic radiologists. Results: The majority, 67%, of the pericarditis cases were evaluated with PE-protocol CTs. Pericardial thickening/enhancement was the most accurate single parameter for pericarditis, with sensitivity of 54–59% and specificity of 91–96%. Conclusion: CT findings, while not sensitive for pericarditis, are diagnostic, with few false-positives. Radiologists should be attentive to pericardial thickening or enhancement on CT studies done for chest pain, as they may be able to suggest pericarditis as an alternative diagnosis for the chest pain.