The decision to accept the case was unanimous among the court's seven justices, although a date for oral arguments has yet to be set. Conservative groups had hoped the court would decline the appeal and allow a lower court ruling against gay marriage to stand.

At issue is a California law passed by voters in 2000 that protects the natural, traditional definition of marriage. Known as Proposition 22, it passed with 61 percent of the vote.

“The people of California spoke in 2000, and the people’s voice should be heard," Glen Lavy, an attorney with the Alliance Defense Fund, said in a statement.

ADF supports allowing the current law to stand.

“Political special interests shouldn’t trump their voice regarding what’s in the best interests of families and children. This is the question the California courts will ultimately be deciding: Who is more important—our children and the voice of the people or politicians and special interest groups?"

Massachusetts remains the only state to recognize gay marriage, although Maryland's highest court heard a gay marriage case in December and could issue a decision in early 2007. Also, Connecticut's highest court is considering whether to accept an appeal of a gay marriage case.

The California Supreme Court's announcement further highlights a divide among conservative groups over a proposed constitutional marriage amendment. Conservatives had hoped to place an amendment prohibiting gay marriage on the ballot in 2006, but could not agree on the proposal's language. They subsequently divided into two competing groups and began gathering signatures. In the end, though, both amendments fell short of the requirement.

That division could prove to be significant. The California court likely will rule on the issue before an amendment can be placed on the ballot. If the court legalizes gay marriage, any statewide campaign to adopt a marriage amendment could be much tougher.

But the court's Dec. 20 announcement also could make it easier to collect signatures to qualify an amendment for the ballot.

The lawsuit was filed by the city of San Francisco and a host of liberal legal groups, including Lambda Legal, the American Civil Liberties Union and the National Center for Lesbian Rights. They won at the trial court level, but lost 2-1 at the appeals court level. That appellate decision was handed down in October.

“Courts simply do not have the authority to create new rights, especially when doing so involves changing the definition of so fundamental an institution as marriage," Justice William R. McGuiness wrote for the majority in October. "... The time may come when California chooses to expand the definition of marriage to encompass same-sex unions. That change must come from democratic processes, however, not by judicial fiat."