Conflict in the Democratic Republic of Congo is killing 38,000 people each month, says the Lancet medical journal.
Congolese hope elections due this year will end their misery
Most of the deaths are not caused by violence but by malnutrition and preventable diseases after the collapse of health services, the study said.
Since the war began in 1998, some 4m people have died, making it the world's most deadly war since 1945, it said.
A peace deal has ended most of the fighting but armed gangs continue to roam the east, killing and looting.
"Congo is the deadliest crisis anywhere in the world over the past 60 years," said Richard Brennan, health director of the New York-based International Rescue Committee and the study's lead author.
"Ignorance about its scale and impact is almost universal and international engagement remains completely out of proportion to humanitarian need,"
Some 17,000 United Nations peacekeepers are in DR Congo, to restore peace and organise elections due by the end of June 2006.
Researchers visited nearly 20,000 households across the country over a three-month period in 2004, recording births and deaths over the previous 18 months.
DR Congo has the world's largest peacekeeping mission
They then compared their results with data from neighbouring countries and before the war began and are confident that their results are accurate.
Children were worst affected by the increased mortality rate, often from easily preventable and treatable diseases like malaria and diarrhoea, the study found.
In some parts, death rates were double the pre-war level, while the mortality rate in the city of Kisangani dropped by 80% after fighting there stopped in 2002.
At its height, at least seven foreign armies were involved in the war.
Many fighters - both foreign and Congolese - have been accused of looting DR Congo's vast natural mineral resources during the war.