It is common for Wilms tumor to invade the inferior vena cava and right atrium, but a massive pulmonary embolism is rare. This case reports describes an 8-year-old male patient who presented to an outpatient clinic with mild pain and abdominal mass. A left renal mass with extension of an inferior vena cava thrombus was seen on abdominal CT, suggesting a Wilms tumor. An exploratory laparotomy biopsy was scheduled, but not completed because the patient's health suddenly deteriorated, and he died despite high-quality cardiopulmonary resuscitation. A massive pulmonary embolism was found to cause a sudden and fatal cardiac arrest.