John Jabbour, a WHO official in Cairo, told AFP that the girl from the southern province of Qena, whom he did not name, was in "very critical" condition. She was being treated with Tamiflu since being hospitalised.
The girl had been in contact with poultry, he said.
The highly pathogenic virus has killed 14 people out of the 34 cases reported to date in Egypt, with children the worst affected. The disease was first diagnosed in the country in February 2006.
Egypt's geographical location on major bird migration routes and the widespread practice of keeping domestic fowl near living quarters have led to it being the hardest-hit country outside of Asia.