HAVANA — Cuba is out to boost its top export, an ever-more critical pillar of its economy. And it's not sugar: hired-out Cuban doctors earn the Communist government over six billion dollars a year.

Medical services exports "are now the leading source of hard-currency income for the nation, and have great potential to keep growing," Foreign Trade Minister Rodrigo Malmierca said at a recent event.

About 40,000 Cubans doctors are working on contracts in 66 countries in Asia, Africa and Latin America.

The next big-ticket client could be Brazil -- the booming South American giant is considering hiring 6,000 Cuban doctors to help cover its health care staff shortage.

It may be somewhat surprising that Cuba -- once known for its exports like sugar, cigars, citrus fruits and rum -- now has four main pillars of its state-controlled economy.

Only one -- nickel -- is a traditional commodity, bringing in $1.1 billion a year as the country's fourth most important import. But in this nation of endless white beaches, nickel is still less than half as significant as tourism, which brings in $2.5 billion a year into government coffers.

Impressively, in a Caribbean country of just 11 million people, remittances -- money sent by relatives abroad -- also end up giving $2.5 billion to the Cuban government. Why? Because their family members here spend it mostly at stores owned by the government.

Each of those big-ticket, hard-currency spinners however pales in comparison to the whopping $6 billion Cuba earns each year in exporting professional services -- sports trainers, teachers and especially doctors -- on overseas contracts.

At home, doctors earn between $25 and $41 dollars each month, and they are not allowed to leave Cuba at will.

If they were to head to the nearby US state of Florida, where Cubans can work legally just after arrival, they could earn a physician's salary of $150,000 per year or more.

Private mechanics, waiters at restaurants catering to tourists, and even hairdressers can earn far more than do highly-trained doctors in Cuba, making overseas medical missions appealing to this country's doctors.

The Cuban government acts as a middleman in the transactions, hiring out their medical staff to foreign countries.

The amounts Havana charges for the services of their workers are not made public, but payment, in hard currency, is for significantly more than what the doctors are paid personally. The government keeps the difference between its contracting price and what it pays the Cuban worker monthly.

It's a big business that is getting bigger: Havana has said it wants to increase the number of countries paying to import Cuban doctors. In a nation proud of its public health tradition, it also is charity in some cases: Cuba gives doctors' services to 40 countries that cannot afford to pay for them for free.

President Raul Castro's government recently vowed that medical missions, which started under his brother Fidel Castro back in 1998, would be maintained and even expanded.

"We are going to continue assistance in solidarity to countries that cannot pay for these medical services, as in the case of Haiti," said Health Minister Roberto Morales.

At the moment, 26 nations are paying the Cuban government to send them doctors, at a salary level determined by the Cuban government.

Morales said money earned from the medical contracting arrangement with Venezuela, for example, "helps pay for expenses we incur in other countries" and is also used "to improve workers' health care and working conditions."

The politically based economic arrangement with Venezuela harkens back to the Cold War, when Cuba exported its sugar to the East bloc for cut-rate Soviet oil.

Cuba hopes one day to be able to tap yet another source of revenue: the cash-strapped government believes there are vast crude oil assets off the island's north coast.

But as long as it is unable to access them, Cuba remains economically isolated and dependent on Venezuela, which provides the communist island with cut-rate oil and is the single largest importer of Cuba's medical export workers, taking in about 30,000 of them.