The American Civil Liberties Union of Illinois, and advocacy group, Lambda Legal, filed the suits for couples who had applied for marriage licenses and were denied.
Illinois defines marriage as a union between a man and woman. However, attorneys say same sex couples should be able to marry under due process, and equality clauses in Illinois' constitution.
In Iowa, a similar lawsuit resulted in the state Supreme Court legalizing same-sex marriage.
Illinois approved same-sex civil unions last year.