A patient was bitten more than a dozen times on his back and neck as he lay sedated in hospital, his parents have claimed.
Jason Ketley, 42, was left with painful and bloody injuries after the rodent attacked him at St Ebba's Hospital, a specialist care unit in Epsom, Surrey.
The alarm was raised when staff spotted Ketley walking around a hospital corridor with the rat hanging off his neck by its teeth. Nurses knocked it off and killed it.
Hospital officials have said it was a field mouse, which the patient's parents have disputed.
"That's an outrageous claim. He had large, open bite marks," Ketley's mother, Pat Boardman, told the Daily Mirror.
"I'm appalled that this sort of thing could happen to my son in an NHS hospital in this day and age.
"He was completely helpless and terrified. It's a disgrace. He was very scared and the staff had to show him they had killed the rat to prove it could no longer hurt him."
Ketley, who has a mental age of two and is bipolar, was taken to Epsom General Hospital where he was given injections against diphtheria, tetanus and polio. His parents made a formal complaint to the Surrey and Borders Partnership NHS Foundation Trust following the incident in November.
The trust's director of quality and nursing, Jo Young, described the incident as "genuinely shocking" and said the couple's concerns were being taken very seriously.