Nearly 300 vulnerable adults in Cornwall and the Isles of Scilly were abused in the past year, the areas' safeguarding adults board said.
In its annual report to the end of March, the board said there had been 536 allegations of abuse and that 55%, about 295, were substantiated.
Only seven cases which were substantiated then led to prosecutions.
Board chairman Rob Cooper said action had been taken to support the victims in more than 80% of cases.
The board is responsible for protecting vulnerable adults from abuse, reducing the risk of abuse and supporting people to stop abuse where it occurs.
It meets four times a year and includes members of Cornwall Council, as well as staff from the NHS, support charities and the police.
Board chairman Rob Cooper, who is independent of the organisations involved, said more prosecutions were not brought forward because of often "complex arrangements and relationships between victims and the perpetrators".
He said: "Most cases of abuse are occurring within people's homes. The perpetrator can either be a member of the family or a carer, either paid or unpaid.
"Whenever we can, we will involve the victim in the investigation.
"We will listen to what they want in terms of an outcome and it is not unusual for people to say that they want some kind of action taken but not necessarily recourse to criminal prosecution.
"Sometimes the abuser is also somebody who is need of care; so, in those circumstances, we would offer both the victim and the abuser support."