Colorado and Washington have become the first US states to legalise the possession and sale of cannabis for recreational use.
The ruling is in defiance of federal law, setting the stage for a possible showdown with the Obama administration.
But another ballot measure to remove criminal penalties for personal possession and cultivation of recreational cannabis was defeated in Oregon, where significantly less money and campaign organisation was devoted to the cause.
Supporters of a Colorado constitutional amendment legalising cannabis were the first to declare victory, and opponents conceded defeat, after returns showed the measure garnering nearly 53 per cent of the vote versus 47 per cent against.
"Colorado will no longer have laws that steer people toward using alcohol, and adults will be free to use marijuana instead if that is what they prefer. And we will be better off as a society because of it," said Mason Tvert, co-director of the Colorado pro-legalisation campaign.
The Drug Policy Alliance, a national advocacy group that backed the initiatives, said the outcome in Washington and Colorado reflected growing national support for liberalised cannabis laws, citing a Gallup poll last year that found 50 per cent of Americans favoured making it legal, against 46 per cent who were opposed.
The outcomes in Colorado and Washington, which already have laws on the books legalising cannabis for medical purposes, put both states in further conflict with the federal government, which classifies cannabis as an illegal narcotic.
The US Department of Justice reacted to the measure's passage in Colorado by saying its enforcement policies remain unchanged. "We are reviewing the ballot initiative and have no additional comment at this time."
Separately, medical cannabis measures were on the ballot in three other states, including Massachusetts, where CNN reported that voters approved an initiative to allow cannabis for medicinal reasons.
Supporters there issued a statement declaring victory for what they described as "the safest medical marijuana law in the country".
Oregon voters rejected a measure to legalise recreational use of cannabis. In Massachusetts voters approved use of medical cannabis, while Arkansas voters rejected it.
In Montana, voters were leaning toward affirming a 2011 law that would scale back a 2004 initiative legalising medical marijuana.
California, whose voters rejected a measure to legalise recreational use in 2010, was the first state to allow medical use in 1996. It is already permitted in 17 states and the District of Columbia.
A measure that would have made Arkansas the first state in the South to legalise cannabis for medical purposes appeared headed for defeat by 51 per cent to 49 per cent with about 80 per cent of the vote tallied.
Under the measures in Colorado and Washington, personal possession of up to an ounce (28.5g) of cannabis would be legal for anyone at least 21 years of age. They also will permit cannabis to be legally sold and taxed at state-licensed outlets in a system modeled after a regime many states have in place for alcohol sales.
Oregon's initiative would have legalised state-licensed sales, as well as possession and cultivation of unlimited amounts of cannabis for personal recreational use.
The Colorado measure will limit cultivation to six plants per person, but "grow-your-own" cannabis would be still be banned altogether in Washington state.
Reuters/Bloomberg