The ruling in California could have an impact on the nationwide debate
California's highest court is to give a long-awaited ruling on the legality of the state's ban on same-sex marriages.
The court's decision may have an impact on the nationwide debate on the issue.
If the judges rule the ban - approved by voters in 2000 - unconstitutional, California could become only the second US state to allow same-sex marriage.
California's legislature has twice passed laws to make gay marriage legal. State Governor Arnold Schwarzenegger has vetoed them each time.
California currently offers same-sex couples who register as domestic partners the same legal rights and responsibilities as married men and women.
Other states, such as Vermont and New Jersey, have similar civil union provisions.
The judges on California's Supreme Court heard arguments for and against the legality of the ban in March. Their decision is due by 10am local time (1800 BST).
The law, approved by Californians in a 2000 referendum and known as Proposition 22, states that "only marriage between a man and a woman is valid or recognised in California".
Geoffrey Kors, executive director of the gay rights group Equality California, told the Associated Press news agency: "What happens in California, either way, will have a huge impact around the nation. It will set the tone."
Equality California joined nearly two dozen gay couples and the city of San Francisco in bringing the case after hundreds of gay marriages conducted in 2004 were ruled invalid.
In early 2004, San Francisco became the first place in the US where gay couples were able to marry after the city's Mayor Gavin Newsom authorised same-sex marriage licences, claiming existing legislation was discriminatory.
In August of that year, California's Supreme Court ruled the mayor had overstepped his authority and nullified the unions.
That action prompted the lawsuit under consideration now.
Ahead of Thursday's ruling, San Francisco chief deputy city attorney Therese Stewart told the AFP news agency that denying same-sex couples marriage rights would make California appear "indifferent" to how gay people are treated.
California Deputy Attorney General Christopher Krueger said he believed there was "a rational basis for the state to adhere to the common and traditional definition of marriage" set out in Proposition 22.
He told AFP that registered same-sex couples were given "all the rights and benefits associated with marriage" under the current laws.