At least 40,000 elderly people in care homes in England are living in "social isolation", according to a survey.
Some 13,000 are completely "without kith or kin" and have no letters, calls or visits at all, says the Relatives and Residents Association (R&RA).
The charity's research, funded by the government, gathered results from 686 of the 10,000 homes in England.
The government said it was "determined to strengthen community action" to help isolated older people.
Judy Downey, chairwoman of the R&RA, said she found the situation "desperately sad".
"We're talking about people who, largely, have dementia," she told the BBC.
"They've lost so much already - they've lost their homes, their families, often their memory. No-one to help stimulate it or remind them of who they were or what they did. No-one who knows their history.
"Some people have never had families, some people have outlived them. Some people may well have families, but their children are on other continents and although they do care about them, they can't possibly be in touch with them."
The R&RA said the two-year study did give an accurate reflection of the wider situation across England despite only 7% of care homes responding.
In fact, it said the estimate of 13,000 living in complete isolation was "conservative".
The charity also warned that these people were at risk of financial abuse or neglect.
It cited one case in which a care home resident's bungalow was bought by the manager for his son - something the charity said was "completely unethical".
Ms Downey said local authorities should take responsibility for finding out how many people in their areas were in this position, and then encourage the creation of "befriending" schemes to recruit volunteers to visit them.
Care Services Minister Paul Burstow said services "designed and delivered in silos" could intensify the isolation and loneliness that older people felt.
"The best care homes personalise the care and support that residents receive," he said. "By understanding their personal history and listening to them, care homes can transform people's lives.
"Caring for older people is everybody's business. That is why the government is determined to strengthen community action. Citizen-led advocacy and community-based befriending services can make a big difference."