A little-known form of collapse, rounded atelectasis, is occasionally seen on chest radiographs. Previous reports are mostly in the German and French literature (Hanke collected more than 50 personal cases). Rounded atelectasis appears as a masslike lesion that often mimics a pulmonary neoplasm. Distinctive features that permit radiographic diagnosis include a rounded or oval shadow 2.5--5 cm in greatest diameter, pleural-based, usually lying along the posterior surface of a lower lobe. The blood vessels near the mass appear to be gathered together in a sheaf as they converge in a curved course toward the mass, much like the tail of a comet. The mass itself is always in contact with a chronically thickened pleura. Five cases of rounded atelectasis have been identified in recent years; three of these are reported here in some detail. Lack of familiarity with this clinically innocuous entity usually leads to a mistaken diagnosis of neoplasm and thoracotomy may be performed.