The research, published this week in The Lancet medical journal, concludes that 4.83 million people died from smoking worldwide in 2000 - 2.41 million in developing countries and 2.43 million in rich nations.
"This study is the first to quantify that the 21st century's 'brown plague' is striking the world's middle- and low-income countries with an intensity equal to that which has already been felt in the world's high-income nations and is, in fact, on the verge of surpassing it," said John R. Seffrin, chief executive officer of the American Cancer Society.
Experts say the study will likely spur governments - especially those in developing countries - to pursue anti-smoking health policies.
Experts have previously estimated tobacco death trends in the industrialized world, where smoking first became prevalent, but evidence from poorer countries has been thin.
The World Health Organization estimated in 1990 that about 3 million people die every year from smoking worldwide, but that was a crude extrapolation of trends in the Western world. Much more has been learned since then about how smoking affects different populations.
A major study in 2001 of smoking patterns in China showed that, unlike in the West, tobacco causes many more deaths there from chronic lung disease than from lung cancer.
A study last month found that in India, smoking mainly kills through tuberculosis rather than lung cancer as in the West.
"Smoking kills people in different ways in different countries, but what is common is this very high toll from smoking, wherever it becomes prevalent," said Dr. Michael Thun, head of epidemiology for the American Cancer Society, who was not involved in the study.
The new estimates, researched by Alan Lopez at the University of Queensland in Brisbane, Australia, and Majid Ezzati of the Harvard School of Public Health, used findings from recent studies to build a global picture.
"Tobacco, which we have traditionally thought of as a Western risk, is really in developing countries now," said Ezzati. "This is the first time, according to these estimates, that there are literally identical numbers of deaths in developing and industrialized countries."
"Much of the increase in smoking in the last few decades has been in developing countries, so we really have shifted tobacco from a Western risk to a global risk, and, more so, a developing country risk. That's where a lot of tobacco control should be moving," Ezzati said.
In an unprecedented global push, the World Health Organization adopted a sweeping anti-tobacco treaty in May.
The so-called Framework Convention on Tobacco Control calls for a general ban on tobacco advertising and promotion - or restrictions in countries where, as in the United States, a total prohibition would violate the constitution.
The treaty says health warnings - including pictures such as diseased gums and lungs - should cover at least half the package. In particular it aims to stop hard-sell tactics aimed at adolescents and strip tobacco of its image of being glamorous and cool.
The accord takes effect after 40 countries have ratified it. Much work lies ahead in trying to put the terms of the convention into practice, especially in developing countries, where 70 percent of the 10 million deaths forecast in 2030 will occur.
Developing countries, already grappling with a heavy burden from infectious diseases, have been at the fore in pushing for the convention, saying they need protection from tobacco multinationals who have switched their sales drives from saturated Western markets to Asia and Africa.
There are about 1 billion smokers worldwide.
Sir Richard Peto, an Oxford University epidemiologist who conducted the studies in India and China but was not involved in the latest global estimate, said he believes deaths in the developing world have probably already outstripped those in rich countries.
"It's difficult because until the studies are done in particular countries, you can't really know," he said.
By Emma Ross