Twenty-two patients who had undergone pneumonectomy for various reasons were evaluated by computed tomography (CT) in order to study the post-pneumonectomy space (PPS). In 13 of 22 cases, the residual PPS contained fluid, even years following surgery. In 9 of 22 cases, the PPS was obliterated. Obliteration of the PPS, one of the events that may follow pneumonectomy, is significantly correlated with the expansion of the nonoperated lung (coefficient, 0.84). Obliteration of the PPS does not present a significant coefficient of correlation with the following parameters, also considered in this study: time interval between surgery and CT, presurgical tests of respiratory function, hemidiaphragm elevation, and retraction of operated hemithorax. The fluid contained in the PPS does not organize but persists or is reabsorbed. Mediastinal shift depends on the expansion of the residual lung. The mediastinum rotates following right pneumonectomy and shifts following left pneumonectomy. The usefulness of CT in the follow-up of the pneumonectomized patient is stressed.