Hospital records belonging to 24 elderly patients were found dumped near a bin by a member of the public.
Steven Murphy discovered documents from University Hospital Coventry outside his flat in Foleshill in March and April.
Hospital bosses have apologised to the patients and their families and have launched an internal investigation.
The Information Commissioner's Office said: "We're aware of an incident and are making enquiries."
Dr Subahjit Basu, an expert on NHS data at the University of Leeds School of Law said it was "a serious incident".
Mr Murphy said: "I saw the hospital numbers, the ages, the names, the full medical conditions they were suffering from, the treatment they were receiving and their consultant's name.
"I thought that's sensitive information. They need to not allow this sort of stuff to get out of the building. They should do random searches of staff if necessary."
University Hospitals Coventry and Warwickshire NHS Trust said it had informed the Information Commissioner's Office itself.
The granddaughter of a hospital patient whose records were among those found dumped near a bin said she was "absolutely devastated".
Jemma Lake said the family had "put their trust" in the NHS to care for her grandfather, who has late stage dementia, but they feel staff "completely let him down".
University Hospital in Coventry says it is investigating how this could have happened.
Posted: Tue May 10, 2016 2:53 pm