The European Court of Human Rights is considering an appeal by the man known as Carlos the Jackal, who says the French authorities violated his rights.
Carlos says he was kept in a small, dilapidated cell for 22 hours a day
The notorious revolutionary, whose real name is Ilich Ramirez Sanchez, is serving a life sentence for the murder of two French agents and an informer.
He claims his rights were violated by being held in solitary confinement for eight years from 1994 until 2002.
He says he was kept in a small cell and only let out for a two-hour daily walk.
A lower chamber of the court ruled last year that the treatment did not violate the European Convention on Human Rights.
It said special measures had been needed to detain a man once regarded as the most dangerous terrorist in the world.
Carlos, a 56-year-old Venezuelan, gained international notoriety as a mastermind of deadly bombings, assassinations and hostage-takings.
He and his right-hand-man Johannes Weinrich closely identified with the Palestinian cause and were supported by the secret services of the former communist eastern bloc.