Normal enlargement of the thymus in infancy can often lead to erroneous clinical suspicion of cardiomegaly. Roentgenographic differentiation is not always definitive but echocardiography is generally effective in differentiating cardiac pathology from an enlarged thymus. In this patient, magnetic resonance imaging was necessary to differentiate benign thymic hyperplasia from pericardial or mediastinal pathology. Thymic involution with a severe neonatal illness, followed by thymic rebound, which later subsided, added to the interest and initial confusion in this patient.