A federal court in California has begun hearing a legal challenge to a ban on gay marriages.
While there have been court cases on gay marriage in the past, they have normally been at the state level, the BBC reports.
This case argues that the ban violates the United States constitution, and could end up being decided at the highest level, the US Supreme Court.
A Supreme Court ruling would determine the fate of gay marriages nationwide, without the possibility of appeal.
The suit, filed by two gay couples in a San Francisco court, challenges Proposition 8 - a ban on gay marriage in the state of California - saying it discriminates against gays.
The law amended California's constitution to restrict marriage to a union between a man and a woman.
Supporters say the constitution enshrines the right to marry, but by limiting it to heterosexual couples it discriminates against gays.
Those opposed to gay marriage, led by Christian groups, say the constitution has always been interpreted as meaning only the right of a man and a woman to marry.
