PATIENT records belonging to people in South Derbyshire could be among 41,000 which have been lost by East Midlands Ambulance Service (EMAS).
The tens of thousands of records are believed to be patient report forms from between September 2012 and November 2012.
The scanned-in forms had been saved on to a data cartridge which has gone missing from the service's Beechdale divisional headquarters in Nottingham.
Service chiefs have moved to reassure patients that it was 'unlikely' that anyone outside of the organisation would be able to read the information.
Sue Noyes, EMAS chief executive, said: "We take our responsibility for the security and confidentiality of the information we record and store very seriously, and express sincere apologies for a patient data loss incident which we have reported to the Information Commissioner this week.
"The cartridge is small and there is a possibility that it is still on our premises; we are conducting a thorough search of the building.
"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."
Anyone who received an ambulance response from September 2012 to November 2012 can speak to the ambulance service on 0115 8845055 between 8am and 8pm.