The justices' move sets aside a lower court's decision in October that upheld state laws banning gays and lesbians from marrying one another. The outcome is not likely until late next year.
Massachusetts is the only state that authorizes same-sex marriage.
The justices are reviewing an October decision by the San Francisco-based 1st District Court of Appeal, which ruled 2-1 that California marriage laws do not discriminate because gays and lesbians get most all the rights of marriage the state confers to heterosexual married couples.
San Francisco Mayor Gavin Newsom put the marriage debate in the national spotlight by allowing same-sex couples to get married at City Hall in 2004.
California's justices halted that wedding spree and voided the 4,037 marriage licenses while sidestepping the core constitutional question. They ruled the mayor did not have authority to make marriage law.
The justices, however, invited a challenge to whether banning same-sex marriage was discrimination - a challenge that reached the court Wednesday after it meandered through the trial and appellate courts.
Whether prohibiting same-sex couples from marrying violates the constitutional rights of gays and lesbians is the biggest question surrounding marriage the California Supreme Court has faced since 1948. That year, seven different justices became the first court in the nation to declare laws banning mixed-race marriages unconstitutional discrimination.
The same-sex case was brought by about 20 same-sex couples and the city of San Francisco. Had the court not taken the case, the lower court's decision would have stood.
San Francisco City Attorney Dennis Herrera said the city is "extremely gratified" the justices are reviewing the case.
"It's perhaps the major civil rights issue of our time," he said.
Randy Thomasson, spokesman for Voteyesmarriage.com, a group opposing same-sex marriage, said he was disappointed with the court's move.
"If the law ain't broke, don't fix it," he said. "This is bad news for marriage and the voters of California who already passed a state law reaffirming that marriage is a natural and beautiful institution between a man and a woman."
The court's recent record on gay and lesbian rights is mixed. In several landmark cases, the justices have concluded that gays and lesbians have the same rights as married couples to sue for wrongful death, to adopt children and to seek child support from former partners.
Last year, the court unanimously ruled, under a sweeping domestic partner law that took effect Jan. 1, 2005, businesses must offer spousal discounts to same-sex domestic partners if it offers those benefits to married couples.
"The Legislature has made it abundantly clear that an important goal of the Domestic Partner Act is to create substantial legal equality between domestic partners and spouses," wrote Justice Carlos Moreno, the court's only Democrat, appointed in 2001 by Gov. Gray Davis.
But in 1998, before Gov. Arnold Schwarzenegger-appointee Carol Corrigan and Moreno took the bench, the seven-member court unanimously sided with the Mount Diablo Council of the Boy Scouts of America, which barred an openly gay man from being an assistant scoutmaster because of his sexual orientation. The justices held that California civil rights laws did not apply to the Scouts, and the Scouts had a First Amendment right to associate with whomever it chose.
The justices, however, were careful not to condone the Boy Scouts. "The resolution of this matter," Chief Justice Ronald George wrote, "does not turn on our personal views of the wisdom or morality of the actions or policies that are challenged in this case."
None of those cases, however, has focused squarely on whether gays and lesbians have a right to marry as barred by 1977 legislation and a 2000 voter-approved measure. In Proposition 22, 61 percent of voters declared marriage as a union between a man and woman.
The closest the court came to deciding the issue was in its 2004 ruling that Newsom overstepped his authority by issuing licenses to gay and lesbian couples. In a 5-2 vote, the court also nullified the marriages.
Justices Kathryn Mickle Werdegar, a 1994 Republican appointed by Wilson, and Justice Joyce Kennard, a Republican appointed by Gov. George Deukmejian in 1989, wrote separately that the court should not nullify the marriages. A decision on that, they said, should await a final ruling on the marriages' constitutionality.
By David Kravets