In this prospective study, thin-section CT was used to diagnose pleural dissemination in patients with pulmonary adenocarcinomas in which the primary lesion was found to be adjacent to the pleural surface by conventional CT. The subjects were 32 patients (25) who had undergone resection and 7 who had not with pulmonary adenocarcinoma without pleural effusion from among the 136 patients with pulmonary adenocarcinoma examined at our institution over 3 years. Thin-section CT images of three regions were obtained, and these included the pleural surface adjacent to the lesion, the minor fissure, and the diaphragmatic dome. Histopathological examination revealed pleural dissemination in 12 patients (5 who had undergone resection and who had not), in 2 of whom a diagnosis could not be made preoperatively. Pleural dissemination was diagnosed by CT in 12 patients; there were 2 false positives. In 6 patients who did not undergo resection, cancerous cells were found in fluid obtained by percutaneous pleural lavage, and mediastinal lymph node metastases or distant metastases were also detected. The diagnostic rate of CT was as follows interlobar space. 100% (7/7); mediastinum, 75% (3/4); diaphragm, 71% (5/7); and chest wall, 57% (4/7). In the patients who underwent resection, sensitivity was 100% for the interlobar space, but it was 0% to 33% for other pleural surfaces.