Hussein wrote to the chief judge on December 5 to say he was no longer willing to attend the trial.
It is unclear what prompted him to appear in court today.
The charges against Hussein and six of his aides are related to the "Anfal" campaign against the Kurds in 1987-1988, in which tens of thousands were killed.
The session started with testimony from a witness, Faiq Muhammad Amid, who worked at a Kurdish militia hospital in 1987. He told the court how Iraqi planes bombed his area with chemical weapons and said that, judging by the condition of those affected, he could tell that mustard gas had been used.
The court has listened to about 80 witnesses so far since the start of proceedings August 21.
Hussein has already been sentenced to death by hanging for the killing in 1982 of 148 people in Al-Dujayl, north of Baghdad. Hussein's lawyers have appealed the death sentence.
(compiled from agency reports)
