Chylous pericardial effusions are extremely rare outside of thoracic and cardiac surgery patients. We report the case of an 8-year-old girl with history of recurrent benign giant cell granulomas who developed a large chylous pericardial effusion with cardiac tamponade soon after beginning therapy with imatinib. In this article, we discuss the presentation, diagnosis, and management and review the published literature of this rarely reported side effect of this medication.