Following a pneumonectomy for cancer, the patients are classically observed by clinical examination and standard chest X-ray. However, torpid empyemas can be missed when they occur after the period of hospitalization and when they are not accompanied by a fever. At the time of postoperative radiotherapy, the dosimetric CT scan constitutes the first examination providing objective information of the endothoracic content. It is therefore necessary on this occasion to assure the normality of the postpneumonectomy pleural space while checking that the substituted liquid is homogeneous and above all that the internal mediastinal part of the cavity has a concave appearance. If that is not the case, an empyema should be suspected. The diagnosis, confirmed by a cytobacteriological examination of the pleural fluid, constitutes a counterindication of the radiotherapy. We present two cases of postpneumonectomy paucisymptomatic empyema which were diagnosed during the course of postoperative radiotherapy when the initial dosimetric CT scan was pathologic and could have allowed an earlier diagnosis.