Bowling for Columbine, the anti-gun documentary by satirist Michael Moore, has won best original screenplay in the Writers Guild of America's awards.
Moore's screenplay was the first documentary ever nominated
This marks the first time a documentary has won the prize. No other documentary had ever been nominated in the 55 years the Guild has been giving awards.
David Hare's screenplay for The Hours won best adapted screenplay at the ceremony widely seen as a good indicator for the Oscars.
The Academy Awards are announced on 23 March.
Screenplays honoured by the Writers Guild often go on to win at the Oscars as well, not least because many of the same people vote for both sets of awards.
Guild officials praised Moore's film as "highly innovative".
"Our members' appreciation of Michael Moore's clever, humorous and personal storytelling is a tribute to his vision and the power of his subject matter," WGA West president Victoria Riskin said.
The film, an essay on America's obsession with firearms, has taken $18m (£11.2m), more money than any other documentary in US history.
The Hours traces the story of three women
It nudged out My Big Fat Greek Wedding, Antwone Fisher, Far From Heaven, and Gangs of New York.
Moore said he was "shocked and honoured" to be nominated for the prize.
"Documentary writing is probably the hardest form of screenwriting because you can't really write until you shoot the film," he said.
The Hours, based on the Pulitzer Prize-winning novel about author Virginia Woolf, beat out Chicago, Adaptation, About Schmidt and About A Boy.
The Writers Guild of America represents writers in the movie, TV and new media worlds.