He is a ''highly intelligent'' boy just 10 months shy of his 18th birthday but he does not have the right to make medical decisions for himself.

The Jehovah's Witness who was ordered to have a life-saving blood transfusion for cancer, despite threatening to rip the IV needle from his arm, is appealing against the court claiming he is mature enough to dictate his own fate.

Supreme Court Justice Ian Gzell overrode the wishes of the boy, known as X, and his parents when he ruled in April that he must undergo treatment for Hodgkin's lymphoma at the Sydney Children's Hospital.

''The sanctity of life in the end is a more powerful reason for me to make the orders than is respect for the dignity of the individual,'' Justice Gzell said. ''X is still a child, although a mature child of high intelligence.''

The boy's barrister, David Bennett, QC, argued in the NSW Court of Appeal on Tuesday that the judgment failed to take into account how mature and competent X was and how close he was to being 18.