US President George W Bush and his Brazilian counterpart have emphasised their countries' common interests despite opposing each other over Iraq and some trade matters.
Lula has impressed Washington since taking office
Luiz Inacio Lula da Silva - commonly known as Lula - was visiting the White House, the first foreign leader who opposed the US-led war on Iraq to do so.
Mr Bush said the relationship between the countries is "vital, important and growing".
The two men announced a series of joint projects ranging from energy to business development in Brazil and fighting Aids in Africa.
"Brazil is an incredibly important part of a peaceful and prosperous North and South America," Mr Bush said as he received Lula for an Oval Office meeting.
The BBC's Steve Kingstone, in Washington, says the left-wing former trade union leader is not a natural political ally of Mr Bush.
He has maintained warm relations with Cuba's Fidel Castro, long a thorn in America's side.
But since taking office, Lula has impressed Washington with a combination of economic discipline and an ambitious long-term programme to combat poverty in Brazil, our correspondent says.
On a personal prospective I am very impressed by the vision of the President of Brazil
Mr Bush said: "This relationship is a vital and important and growing relationship.
"On a personal prospective I am very impressed by the vision of the President of Brazil. He not only has a tremendous heart, but he has got the abilities to encourage prosperity and to end hunger."
For his part, Lula spoke of a great partnership but it should, he said, be based on sincerity and trust going beyond a few occasional photo opportunities.
The dynamic between the two men is probably the key to this relationship for the next few years, our correspondent says.
Lula's domestic reforms have provoked trade union protests
Together the two governments are chairing negotiations towards a hemisphere-wide free trade agreement scheduled to come into force in 2005.
The United States is the largest investor in Brazil, with 400 firms and investments of $30bn (£20bn), while Brazil exports some $15bn (£10bn) worth of goods to the US.
With a population approaching 175 million, Brazil is the second largest country in the Americas, after the US - and South America's largest economy.
Lula would like to ensure greater access for Brazil's huge agricultural sector to US markets before agreeing to any trade deals.
Meanwhile, American policymakers are increasingly looking to Brazil for help in resolving some of the most difficult issues between the hemispheres.
These include the confrontation between US companies and Venezuela's Hugo Chavez, and the drug-trafficking issues in the Andes states.
The Brazilian president is accompanied by no fewer than 10 cabinet ministers, in the biggest Brazil-US summit since World War II when President Frank D Roosevelt persuaded Brazil to join the war effort.