A woman was told she had cancer by a hospital employee who accessed her record days before she was due to receive test results from her consultant.
Pippa Boardman, from Lostwithiel in Cornwall, said she had lost her dignity and privacy through the data breach at the Royal Cornwall Hospital in Truro. She said she did not receive an apology from the hospital until it was contacted by the BBC.
The trust said it was "truly sorry" and "appropriate action had been taken".
It added: "While we cannot disclose the details of individual employment matters, we want to reassure Ms Boardman that we have treated this extremely seriously."
Boardman, who is now recovering well from cancer, said her personal information was accessed by a man she vaguely knew who worked at the hospital.
She said: "I got a phone call at home from somebody I know through the sport that I play.
"This man said to me 'Pippa, I don't want you to worry, but you've got cancer'.
"I just couldn't really believe it".
Two days later Boardman was told by her consultant she did have breast cancer.
Nine months on, she said her strong recovery did not undo the damage done by the experience, adding: "I lost my dignity and I've lost my privacy."
Along with Boardman's case there have also been other recent NHS data breaches, described by Health Secretary Yvette Cooper as a disgrace.
Boardman said part of her trauma had been caused by the Royal Cornwall Hospital's handling of the case.
She said: "It really distresses me people can't see just how invasive and intrusive it is if somebody looks at your information and then shares it inappropriately."
She said the trust told her the breacher's rights meant it could not give her information, and it only apologised after being contacted by the BBC.
She said: "It took a lot of courage to complain... and I didn't understand why the hospital wasn't really prepared to engage with me.
"I understand that it's because his rights have to be protected. But I kept thinking where are my rights in this."
Boardman said she wanted to prevent others going through a similar experience.
"I'm in a very good place in terms of my cancer. I'm just in an awful place in terms of what happened and how it happened," she said.
The Royal Cornwall Hospital Trust said it was "truly sorry" about how Boardman was contacted.
It said: "Whilst the member of staff had legitimate access to her records as part of their work, it was outside of their role to contact her.
"This should not have happened and we recognise the distress and concern it will have caused.
"Appropriate action was taken following our investigation which was carried out immediately after Ms Boardman raised her concern."
The trust said it had reminded staff records "must only ever be accessed for a legitimate work-related reason" and it was also reviewing policies "in light of the strengthened NHS England guidance on inappropriate access to patient information".