We discuss an atypical presentation of a pericardial cyst appearing as a mobile lesion extending into and out of the right pulmonary major fissure. With the patient in the supine position, the cyst mimicked a pleural pseudotumor along the right oblique fissure. In the left lateral decubitus position, the cyst migrated to the right cardiophrenic angle and assumed an appearance more characteristic of a pericardial cyst. At surgery, a stalk was discovered attaching the cyst to the pericardium explaining its migrating nature. We conclude that computed tomography scanning in the decubitus position is useful for differentiating a mobile pericardial cyst from a pleural pseudotumor.