All five animals at Malaysia's Sungai Dusun rhinoceros centre have succumbed to what is believed to be acute blood poisoning following bacterial infection.
The deaths throw a bleak spotlight on the state of the world's most critically endangered rhino species, and raise questions about Malaysia's captive breeding effort, yet to bear fruit after two decades.
The country has got only two rhinos left in captivity - in Sabah state on Borneo.
Habitat loss and the poaching of animals for their horns and skin - usually for aphrodisiac concoctions - have taken a terrible toll on the Sumatran rhino, Dicerorhinus sumatrensis.
Only about 300 survive in the wild in Southeast Asia.
Although Javan rhinos, Rhinoceros sondaicus are rarer, wildlife experts say their numbers have steadied at about 60 in Indonesia and a few in Vietnam.
"The populations in Java are stable, they have not lost any for quite a while," says Chris Shepherd, who tracks illegal trade in animals and plants with TRAFFIC Southeast Asia.
Shepherd says the rhinos' plight is shared by other species in the region.
"Things are still in serious decline. They have not solved it yet, I see the rhinos as one of many," he says, adding that the key is policing the areas endangered species still inhabit, such as peninsular Malaysia's Taman Negara national park.
Only two Sumatran rhinos are now
only left in capitivity in Sabah
"The highest priority is much stiffer enforcement around the protected areas and stiffer penalties for anyone caught poaching, trading or using rhino products."
Southeast Asia is home to two of the world's five remaining rhino species, the other main populations being in India and East and Southern Africa.
Rhino vital statistics
Sumatran rhinos, small, hairy creatures standing about 1.5 metres at the shoulder, suffer for their love of solitude and dense, jungle habitat.
The larger rhinos in Africa and India generally inhabit open grassland or scrub bush, making them easier to find.
"In Africa, you pay to go and see rhinos and you see them. That doesn't happen here," says Shepherd.
Ecotourism is increasing in Southeast Asia, but has yet to provide the hard-currency lifeline thrown to African countries. That means captive breeding may be critical.
Thomas Foose, a rhino expert with The World Conservation Union, says the recent deaths should not stop breeding efforts.
"A captive propagation programme is a vital component as basically an ultimate insurance policy for the species.
"We are certainly not going to abandon the propagation programme because, with too many other species, the captive programmes have worked," he said, pointing to animals such as the Californian Condor and the Arabian Oryx.