The details of 800 patients of East Surrey Hospital have gone missing after an unencrypted memory stick was lost.
According to a report in the Crawley Observer information such as the details of operations, names and dates of birth were lost in September last year, but never reported.
The Surrey and Sussex Healthcare NHS trust, which the hospital is part of, revealed the loss in a report in its latest annual report.
Michael Wilson, CEO at the Surrey and Sussex Healthcare NHS trust, said all staff should always use encrypted memory sticks when transferring patient data. "It is regrettable that this didn't happen on this occasion and the member of staff has been taken through the Trust's disciplinary procedures and has received further training."