The personal details of more than 200 cancer patients have been lost by the health trust where 90 people died from the clostridium difficile superbug.
The memory stick went missing from this office at the hospital
A memory stick containing the names and dates of birth of 244 patients at the Peggy Wood Breast Care Centre, and some clinical details, has been mislaid.
The Maidstone and Tunbridge Wells NHS Trust said it had contacted all the people concerned to reassure them.
It said the memory stick did not hold patient addresses or GP details.
Professor Roger James, the trust's deputy medical director, said: "We are very sorry about the loss of this information and any concern it may cause to any of the patients.
"We wrote to them all on Friday 7 December and followed that up with a telephone call over the weekend, and on Monday and Tuesday, so that we would be able to explain the situation fully and answer any of the patients' questions.
"People were appreciative that we had let them know what had happened, and that we had phoned them.
"A few were concerned that their medical notes might be missing and we were able to reassure them that this was not the case."
The trust said a search of the area where the computer memory stick was kept was carried out immediately, and it was reviewing its IT systems to ensure the situation could not arise again.
'Of little use'
Prof James said: "Other patients of the Breast Care Centre need not be concerned - we know who the people involved are and have contacted all of them.
"We believe that the memory stick was misplaced in the centre and it may still turn up. The information would be of little use to anyone who is not a health care worker.
"This device was used simply as an extra back-up for a limited amount of information and patients' full notes are kept on our normal systems, so there is no impact at all on patient care."
On Wednesday, health inspectors started a two-day visit at the trust as part of follow-up procedures after a damning Healthcare Commission report.
The report said C.diff was probably the main cause of death for 90 patients at the trust's hospitals.