At least 500 million cases of malaria occur each year - nearly 50% more than estimated by the World Health Organisation (WHO), health experts say.
Not all mosquitoes carry the malaria parasite
The findings of an international team of specialists in tropical medicine were published in the journal Nature.
The experts pointed out that the WHO relied heavily on clinical reports of the disease for its statistics, but many sufferers did not seek treatment.
They voiced particular concern over the situation in south and South East Asia.
The scientists from Kenya, Thailand and Britain said the threat from the deadlier form of malaria - due to the parasite plasmodium falciparum - had been hugely underestimated in the region.
The team also produced a map showing malaria hotspots, which it says should help the WHO campaign to halve deaths from the disease by 2010.
"Getting numbers right is important," said Professor Bob Snow of the Kenyan Medical Research Institute in Nairobi, who is the co-author of the report.
"Not knowing the size of the problem limits our ability to articulate how much money we need to tackle the problem.
Mosquito-borne parasitic disease
Endemic in dozens of developing tropical countries
Kills about 1m people each year
Symptoms: fever, headache, vomiting
Incubation period: about 9-14 days
No vaccine, treatment by drugs
"This is particularly important for new drugs needed to fight malaria. These are expensive and difficult to produce," Prof Snow said.
His words were echoed by Richard Feachem, executive director of the Global Fund to Fight Aids, Tuberculosis and Malaria.
"Many have believed that existing data grossly underestimates malaria, morbidity and mortality in Africa and Asia. We now have confirmation of this," Mr Feachem said.
The WHO estimates the global incidence of malaria at about 300m every year, with 90% of cases occurring in Africa.
The UN agency says more than a million people are killed by the disease each year.