An NHS trust has lost a computer disk containing details of 42,000 ambulance patients.
The disk contains copies of handwritten report forms listing anyone who used East Midlands Ambulance Service (EMAS) over two months in 2012.
EMAS chief executive Sue Noyes said the incident had been reported to Nottinghamshire Police and the Information Commissioner.
She said the disk has been locked away at its headquarters in Nottingham.
Ms Noyes added there had been no break-in at the Beechdale building.
The records only contained names and addresses of patients treated by paramedics at the scene of an incident between September and November 2012.
Ms Noyes has apologised for the loss of patient data but said it would be very difficult for the information to be accessed.
She said: "We are certain the data can only be read via specific hardware which we have in our premises and which is no longer in production - i.e. it is obsolete.
"Therefore it is unlikely that the information stored on the missing cartridge can be viewed by anyone outside of the organisation."
She said it was likely the disk had been lost within the headquarters and a search of the building was taking place.
Nottinghamshire Police confirmed that it had been asked to help investigate the loss of the disk.
Patients have been told they can contact EMAS with any concerns they might have.