Conflict in the Democratic Republic of Congo is killing 38,000 people each month, says the Lancet medical journal.
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.