PURPOSE: The purpose of our study was to identify the CT findings that help detect pleural dissemination from lung cancer and to evaluate the usefulness of selected diagnostic criteria. MATERIALS AND METHODS: After a computerized database search of 606 patients who had undergone thoracotomy for primary lung cancer, 23 patients were identified as h aving surgically documented pleural dissemination. From the same database, 50 patients without pleural dissemination during thoracotomy were randomly selected as controls. Preoperative CT scans and medical records were rev i ewed retrospectively, and findings were compared between the two groups. RESULT: One or more of three types of pleural thickening (plaque-like, nodular, and fissural) were identified on CT as the most discriminating finding (sensitivity, 74 % ; specificity, 60 %; p = 0.007). The following findings were also significantly discriminating (p<0.05): contiguity of primary tumor with the pleural surface as seen on CT; adenocarcinoma in cell type; and a peripheral tumor defined as one in which bronchoscopy revealed no endobronchial lesion. The use of combinations of these findings in addition to pleural thickening rendered diagnostic criteria more specific at the cost of the sensitivity. CONCLUSION: During preoperative CT evaluation of lung cancer, the recognition of subtle pleural thickening helps detect pleural dissemination. The likelihood that subtle pleural thickening represents pleural dissemination is increased when a primary tumor is contiguous with the pleural surface, is an adenocarcinoma, or is peripherally located.