Hyperleukocytosis in leukaemia causes leukostasis, which can lead to thrombosis of arterial systems of various organs and cause renal and retinal thrombosis, acute limb ischemia and myocardial infarction. We report a case of an eight-year-old male child who had pericardial thrombus formation consequential to hyperleukocytosis. Keywords Hyperleukocytosis, Leukostasis, Pericardial thrombus, Leukemia