We describe the use of computed tomography (CT) in diagnosing and documenting diaphragmatic paralysis in a patient with right lung cancer that invaded the mediastinum. The patient was unable to breath hold during CT scanning, and the images were degraded by motion artifact. Motion artifact, however, was noted only in the left lung. In patients with suspected phrenic nerve dysfunction who are having chest CT for any reason, we suggest obtaining several dynamic images at the lung bases during forced exhalation. The lack of motion artifact in the lung parenchyma may be useful for diagnosing phrenic nerve injury and hemidiaphragm paralysis.