The objective of this study was to describe CT findings and to determine the diagnostic value of CT in diagnosis of acute mediastinitis. CT findings were retrospectively studied in 40 patients with suspected acute mediastinitis, including 28 postoperative patients, five with acute descending necrotizing mediastinitis (ADNM), and seven with suspected post-traumatic perforation of the esophagus. Findings included increased attenuation of mediastinal fat (100%), localized mediastinal fluid collections (55%), free gas bubbles in the mediastinum (57.5%), mediastinal lymph nodes (35%), pericardial effusions (27.5%), pleural effusions (85%), lung infiltrates (35%), sternal dehiscence (40%), and pleuromediastinal fistula (2.5%). The sensitivity and specificity of CT in postoperative patients in the first 17 days was 100% and 33% respectively, and after day 17, 100% and 90%. In patients with ADNM sensitivity was 100% while in patients with suspected esophageal perforation sensitivity and specificity were 100%. CT is a highly sensitive technique for the detection of mediastinitis of various causes. For the postoperative patients there is clear time dependence for CT interpretation and accuracy. In patients with suspected ADNM, and traumatic esophageal perforation CT is highly specific early after clinical presentation.