A computer disc containing the details of 42,000 ambulance patients in the region has been lost.
The disc is believed to contain electric copies of scanned handwritten report forms of patients who used East Midlands Ambulance Service (EMAS) from September 2012 to November 2012.
The small cartridge has gone missing from the service’s Beechdale headquarters in Nottingham.
A service spokesman said there is a possibility that it is still on the premises and that a thorough search of the building was underway.
The trust said it “had a duty to inform people” of the incident.
Sue Noyes, chief executive at EMAS, said: “We take our responsibility for the security and confidentiality of the information we record and store very seriously.
“We express sincere apologies for this incident which we have reported to the Information Commissioner this week.
“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.
“Therefore it is unlikely that the information stored on the missing cartridge can be viewed by anyone outside of the organisation,” she added.