OBJECTIVE. This study describes the atypical presentation of atelectasis as a pseudomass inpatients with severe bullous emphysema. MATERIALS AND METHODS. The radiologic studies and clinical records of 11 patients with severe bullous emphysema and masslike opacities representing atelectatic lung were reviewed. The diagnosis was proven by surgery in nine patients and by follow-up imaging in two patients. RESULTS. Pseudomasses were found primarily in the upper lobes and the right middle lobe (n = 10). Common features included an oblong, lenticular, or triangular shape (n = 8); a sharp interface with aerated lung (n = 9); hyperexpansion of the hemithorax containing the pseudomass (n = 8); a central location abutting the mediastinum (n = 8); and adjacent emphysema (n = 11). CT revealed subsegmental atelectasis in other lobes adjacent to the bullous lung in six patients. In three patients, the pseudomasses were associated with persistent distention of large bullae following spontaneous pneumothoraces. Reexpansion of pseudomasses occurred in seven of the eight patients who underwent resection of bullous lung. CONCLUSION. These pseudomasses most likely result from compression of the lung by adjacent large bullae. The diagnosis should be suspected when central, sharply marginated, masslike opacities that are oblong, lenticular, or triangular are bordered by severe bullous emphysema.