Darren Millar said he was greatly concerned about the discovery
An NHS Trust is to investigate after an AM found patient documents in the communal area of a block of flats.
Darren Millar, Tory AM for Clwyd West, noticed the papers when collecting mail from the apartment he uses in Cardiff Bay when the Welsh assembly sits.
He said the documents were on top of black bin bags and referred to patients and medical students.
Cardiff and Vale NHS Trust said it could only investigate fully when Mr Millar contacted them with information.
Mr Millar said the names, addresses and dates of birth of patients were visible among to 40 to 50 sheets of paper.
Doctors' details and bleeper numbers were on some of the documents.
I would be pretty cheesed off if they were my records and they had been found in the rubbish
A dictating machine cassette was also left with next to the lifts and communal rubbish chute.
Mr Millar said: "There are a number of patient records, copies of peoples' ECGs (electro-cardiograms), details of blood tests and conditions in the past.
"I would be pretty cheesed off if they were my records and they had been found in the rubbish.
"It was just inside the hallway and it had clearly been left out for someone to take and chuck in the rubbish. There had been no attempt whatsoever to hide these records."
Mr Millar said he would hand over his find to Cardiff and Vale NHS Trust and ask them to investigate.
He said he was greatly concerned the documents had been left in the light of recent revelations about papers from UK government departments going missing.
"It seems the material has some connection with a consultant paediatrician.
"Obviously it is entirely inappropriate that they should be on view like this in a public place," he told the Western Mail.
"It would also be extraordinary if confidential documents of this kind were being taken home by medical or other staff."
Cardiff and Vale NHS Trust Medical Director Ian Lane said: "We take any issues surrounding mislaid patient information very seriously.
"Unfortunately we cannot yet investigate fully because Mr Millar has yet to contact us with the information. When he does, we will conduct in-depth inquiries."