International Aids campaigners have broadly welcomed a $15bn plan by the United States to fight the disease in Africa and the Caribbean.
The legislation needs to be approved annually by Congress
US President George W Bush signed the emergency plan into law, describing the need to take steps against the disease as among the most urgent requirements of the modern world.
The legislation will nearly triple US contributions towards fighting Aids and provides funding over the next five years.
Rock star and developing world campaigner Bob Geldof is among those who have praised the measure.
However, opposition Democrats in the United States have warned much of the money might not be available, and many questions remain about how the plan will be implemented.
'Step in the right direction'
Uganda, which has lost more than 900,000 people to Aids-related illnesses since the disease was first diagnosed in the country since 1982, backed the US plan.
A spokesman for Ugandan President Yoweri Museveni said the money would be a great boost to Ugandan efforts to combat the disease.
In the Caribbean, a senior Aids worker, Dr Yitades Gabri, also welcomed the move.
"This is actually a step in the right direction," Dr Gabri, chairman of the Caribbean Coalition of National Aids Programme Co-ordinators told the BBC.
The head of a joint United Nations programme on HIV and Aids, Peter Piot, said the money could dramatically reduce deaths from the disease that has killed more than 20 million people.
Surprising aid and family planning organisations, Bob Geldof had warm words for President Bush.
"You'll think I'm off my trolley when I say this, but the Bush administration is the most radical - in a positive sense - in its approach to Africa since Kennedy," Mr Geldof, who organised the 1985 Live Aid fund raising concert for Ethiopia, told Britain's Guardian newspaper.
At a signing ceremony at the State Department, President Bush said Aids was filling graveyards, creating orphans and leaving millions in a desperate fight for their own lives across Africa.
Mr Bush said his country had a moral duty to act - and he called on Europe, Canada and Japan to follow Washington's example.
"I will remind them that time is not on our side. Every day of delay means 8,000 more Aids deaths in Africa and 14,000 more infections," Mr Bush said.
The plan has come under fire from some quarters in Washington angry that a third of the money spent on prevention must be used for projects promoting abstinence rather than safer sex.
The new package recommends that 55% of direct aid should go to treatment programmes, 20% to prevention, 15% to care for those dying of Aids and 10% to children orphaned by the disease.
The legislation must also be approved annually by the US Congress.