An investigation is under way into the incident
An NHS trust has apologised after a computer memory stick, containing the confidential files of 200 patients, was found in a street.
Tees, Esk and Wear Valleys Trust said the stick was found by a member of the public in Barnard Castle, Co Durham.
It stored a summary of medical histories and patients' national insurance numbers and addresses.
The trust confirmed the stick was lost by a computer technician upgrading computers in Teesdale and Weardale.
Trust chief executive Martin Barkley said an investigation was under way into the "isolated incident".
He said: "There has been a serious breach of these policies and of patient confidentiality.
"We are very sorry this has happened but grateful that it has been brought to our attention.
"We have already written out to all staff to remind them of their responsibility to safeguard patient information and to follow trust procedure. "
He added early investigations had also confirmed that a number of staff stored confidential data on their hard drives, contravening trust policies on information security.
The trust took over services run by the former County Durham and Darlington Priority Services NHS Trust and Tees and North East Yorkshire NHS Trust in April 2006.