Medical records of more than 5,000 patients have been stolen from a GP practice in Downpatrick, Co Down.
The break-in happened at Christmas at the Pound Lane surgery, with computer hard drives and encrypted DVDs taken, a spokesman for the South Eastern Health and Social Care Board said.
He said it was unlikely thieves could access the information because of security features.
He said: "The practice and the board have been working in the period since the break-in to establish whether the stolen information could be retrieved or recovered from other sources. However, this has not proved possible."
The board said the theft involved records of around 5,400 patients which are being recovered manually in a process that could take several months.
Information on the hard drives is protected by separate security features which make it extremely unlikely that thieves could access it, even with specialist computing knowledge and equipment.
Information on the back-up DVDs is encrypted - making it almost impossible that thieves could access it.
The spokesman added: "Any impact on patients is being minimised by the practice, which has contacted patients by letter to advise them of the steps being taken and how they can assist in having their records restored as soon as possible.
"The practice has also been working with other healthcare providers in the Downpatrick area, including pharmacists, to ensure that prescriptions continue to be dispensed as normal."
SDLP health spokesman Conall McDevitt said investigations should not be left to the board.
"People across the north are entitled to reassurance about security features to prevent unauthorised access. But there is also the issue of back-up disks being kept at the same location as the original information," he said.
"It is very serious that the board has been unable to reconstruct important patient information from other sources."
A police investigation is continuing into the theft of the records.
© Press Association