The California Supreme Court is to announce its decision on whether Proposition 8, the initiative that banned gay marriage in the state, is constitutional.
The decision is to be announced at 10am PST, (6pm GMT).
Gay marriage was legalised in California in summer 2008. However, Prop 8 defined marriage as being between a man and a woman last November. Supporters of gay marriage say the initiative is unconsitutional and discriminatory.
The issue is over whether a voting majority can overrule minority rights previously recognised by the court, as it had previously declared that gays and lesbians have a constitutional right to marry.
Eighteen thousand couples rushed to tie the knot during the time gay marriages were legal in the state. The court must also decide whether to uphold those marriages.
Vigils and marches have been planned for this evening to either celebrate the ruling or protest against it.
Connecticut, Massachusetts, Vermont, Iowa and Maine now allow gay couples to marry.