Rounded atelectasis is an unusual from of peripheral lober collapse. It appears as a mass-like lesion on chest radiography that often mimics a pulmonary neoplasm. Four cases of rounded atelectasis were diagnosed radiologically, and the CT findings were analyzed. In every case the CT showed a round mass, 3-5cm in diameter, abutting a thickened pleural surface on the lung periphery. The bronchovascular bundles near the mass appeared to converge on the mass in a curved course, forming a so-called "comet-tail" appearacne on the CT scan. Our experience suggests that the CT findings of rounded atelectasis are characteristic of the disease and that the diagnosis of the disease can be made radiologically without histological confirmation.firmation.