At least one quarter of the world's mammals are at risk of extinction, according to this year's Red List of Threatened Species.
The International Union for the Conservation of Nature, which publishes the list, says the environmental crisis will affect more people than the current financial crisis.
Populations of more than half of mammalian species are falling, with Asian primates particularly at risk. The biggest threat to mammals is loss of habitat, including deforestation.
But there is good news for the African elephant, whose recovery leads to removal from the high-risk list.
This year's Red List looks at 5,487 mammals, and concludes that 1,141 are currently on the path towards disappearance.
This may be an under-estimate, the authors caution, as there is not enough data to make an assessment in more than 800 cases. The true figure could be nearer to one-third.
"Within our lifetime, hundreds of species could be lost as a result of our own actions, a frightening sign of what is happening to the ecosystems where they live," said Julia Marton-Lefevre, director-general of the International Union for the Conservation of Nature.
"We must now set clear targets for the future to reverse this trend, to ensure that our enduring legacy is not to wipe out many of our closest relatives."
The report's authors said the current concern with financial matters must not be allowed to obstruct the decline in the Earth's natural systems.
"The financial crisis is nothing compared with the environmental crisis," the deputy head of IUCN's species programme, Jean-Christophe Vie, told the BBC.
"It's going to affect a few people, whereas the biodiversity crisis is going to affect the entire world. So there is a risk that because of the financial crisis, people are going to say 'yeah, the environment is not that urgent'; it is really urgent."