A diffuse, bilateral interstitial infiltrate is the most common radiographic finding in Pneumocystis carinii pneumonia (PCP) in patients with the acquired immunodeficiency syndrome (AIDS). However, atypical roentgenographic patterns also exist. Chest radiographs of 100 consecutive AIDS patients with PCP were retrospectively analyzed for the presence of pulmonary air-filled cysts, or pneumatoceles, which were identified in 10% of the cases. The pneumatoceles were typically thin-walled with no intracystic material and no predilection for a particular area of the lung. They appear to behave like pneumatoceles due to other infectious processes. They were present on the initial radiograph or developed during treatment of PCP. Analysis of available follow-up radiographs indicated resolution of the pulmonary cysts within 7 months in most cases. The cause of these cysts is unknown, but a "check-valve" obstruction or, perhaps less likely, parenchymal necrosis may be involved. Rupture of these pulmonary cysts may lead to spontaneous pneumothorax.