Pulmonary embolism can be observed at any age, but it occurs very rarely in the infant. The authors report the case of a 12-month-old boy who suddenly died during clinical improvement of gastroenteritis associated with otitis. Autopsy showed signs of dehydration and acute left pulmonary embolism, which was the cause of death. Thrombosis of the inferior vena cava extending into renal veins was seen. In this case, plasma hyperosmolality caused by dehydration might have favored the formation of venous thromboses. The various risk factors of pulmonary embolism in the child are discussed, including genetic factors.