Law enforcement sources tell us Leo is being "extremely cooperative" with prosecutors on the Aretha Wilson case. 40-year-old Wilson appeared, briefly, in court on Wednesday and is being held on $150,000 bail.
According to our sources, Leo has told authorities that he will take the stand and testify against Wilson, if necessary. Wilson allegedly slashed DiCaprio's face with a glass during a 2005 party in the Hollywood Hills.
Wilson was extradited from Canada last week and charged with assault with a deadly weapon.