The supreme court in Massachusetts has ruled that same-sex couples must be given equal marriage rights in the north-eastern American state.
The issue came to court after seven gay couples filed a lawsuit
It bolsters a ruling by the same court that gay couples have a constitutional right to full marriage rights, not just those conferred by a civil union.
That first ruling set off a firestorm of controversy in the US.
It led the White House to promise to defend what it calls the "sanctity of marriage".
The BBC's Jane Standley in New York says the Massachusetts decision is a highly significant milestone and comes in a year which has seen more legal recognition for gay rights in the United States and in its neighbour, Canada.
But the ruling came a day after the legislature in the state of Ohio gave final approval to one of the most sweeping bans on same-sex unions in the country.
This makes Ohio the 38th state to prohibit the unions.
But the law there goes further by barring state agencies from giving benefits to non-married couples irrespective of orientation.
Initial ruling vague
The Massachusetts Supreme Judicial Court issued its ruling in response to a request from the state senate about how it must fulfil its new legal obligations to same sex couples wanting to get married.
"The history of our nation has demonstrated that separate is seldom, if ever, equal," the four justices who ruled in favour of gay marriage wrote in the response.
The initial ruling by the court in November, prompted by a lawsuit by seven gay couples who were refused marriage licences, had been vague in its wording.
Activists on either side of the debate argued that it did not make clear if a civil union - which confers many of the same rights as marriage but not the title - would suffice.
Now it is clear that it will not - marriage means marriage, and the state legislature has six months to change the law to make it happen.
Analysts say that the first same-sex ceremonies in Massachusetts could take place as early as May.
The initial ruling was immediately denounced by President George W Bush, who promised to pursue legislation to protect the traditional definition of marriage as being between couples of different sexes.
He reiterated the point in his State of the Union address two weeks ago, prompting speculation that he will introduce an amendment to the constitution to essentially override the Massachusetts ruling.