Thin-walled pulmonary cystic lesions were found in five immunocompromised patients, four with acquired immunodeficiency syndrome (AIDS). Four patients had Pneumocystis carinii pneumonia (PCP), and one had pulmonary lesions and disseminated P carinii infection. Two patients demonstrated P carinii within necrotizing, thin-walled, smaller intraparenchymal cavities lined by organisms, exudate, and chronic inflammation. Larger, typically apical and subpleural cysts, lined by fibrosis and/or alveolar parenchyma with little inflammation, were also found during acute episodes. The larger subpleural cysts can arise via rupture of intraparenchymal necrotizing cavities into the subpleural area. Pneumothorax in the four patients with AIDS could not be cured by close thoracostomy drainage; all required pleurodesis. The cysts persisted in cases that were followed up. All cysts were more obvious and numerous with computed tomography (CT), especially with 1.5-mm collimation. CT may be indicated in immunocompromised patients with unexplained pneumothorax or when tube thoracostomy has failed and surgery is being considered, as it can positively influence the operative approach.