For preoperative evaluation of chest wall and mediastinal invasion by lung cancer, computed tomography (CT), combined with artificial pneumothorax (pneumothorax CT), was performed in 43 patients with lung cancer in whom conventional CT scans showed that the mass was contiguous to the chest wall (n = 30) and/or mediastinum (n = 25) but without evidence of definite tumor invasion. Invasion was diagnosed on the basis of whether an air space existed between the mass and the adjacent structures. In three patients pneumothorax was not produced. After the procedure, four patients developed symptomatic pneumothorax, and one, subcutaneous emphysema. Comparison of diagnoses based on findings at pneumothorax CT, surgery, and pathologic examination showed that pneumothorax CT is 100% accurate for chest wall invasion and 76% accurate for mediastinal invasion. The authors conclude that this procedure is helpful in accurate evaluation of the T criterion in lung cancer, especially for patients in whom findings at conventional CT suggest tumor invasion of the chest wall and mediastinum.