To determine the presence of chest wall and mediastinal invasion by lung cancer and to establish the origin of chest tumors, we studied 12 patients with intrathoracic tumors by using chest CT combined with artificial pneumothorax. Six patients had primary lung cancer, two had metastases, and one each had neurofibroma, pericardial cyst, chondroma of the rib, and malignant mesothelioma. All 12 tumors abutted the chest wall or mediastinum and could not be separated by conventional CT. Between 400 and 800 ml of air was injected into the pleural space before a second CT scan was obtained. No invasion was found at surgery in cancers that were separated from chest wall or mediastinum on CT scans. Surgery revealed chest wall invasion in three patients in whom the CT scans showed that the tumor was not separated from the chest wall. Only one patient with a tumor that was not separated from the mediastinum on CT did not have mediastinal invasion: in this case, only adhesions were found at surgery. Thus, in the eight patients with primary lung cancer and metastasis, sensitivity, specificity, and accuracy were 100%, 80%, and 88%, respectively. In four patients with mediastinal or pleural tumor, CT combined with pneumothorax was useful for establishing the origin of tumors. In all, 11 of the 12 patients were correctly evaluated by using this method. No complications occurred, except for mild chest discomfort in one patient. This study suggests that chest CT combined with artificial pneumothorax is useful for the evaluation of the extension of lung cancer into the chest wall and mediastinum and for the diagnosis of the site of origin of intrathoracic tumors.