While rounded atelectasis (RA) is considered to be rather common in the United States and Europe, the total number of RA cases reported from Japan still remains approximately 30. We have long been aware that there are many variations in the radiographic appearance of so-called RA and that RA has never been clearly defined. We retrospectively reviewed 22 cases collected as RA and its variants from several institutions. We defined RA as "peripheral atelectasis mimicking tumor secondary to shrinkage or bending of the pleura of various degrees, and accompanied by lung distortion." The diagnostic criteria of typical RA include (1) peripheral tumoral shadow in contact with pleural effusion or thickened pleura, (2) acute angle between the pleura and the shadow, (3) convergence of the pulmonary vessels and bronchi and (4) volume loss of the affected lobe. However, there are cases which lack some of these criteria but are considered to be included in the broad category of RA. We propose that RA should be considered to be an entity having a wide spectrum. Typical lesions showing "cranial tilting" of Hanke are on one side of the spectrum and small linear or strand shadows extending from the thickened pleura are on the other.