Details of nearly 900,000 emergency calls to the ambulance service have gone missing while in transit, the Scottish Ambulance Service said on Monday.
The records of 894,629 calls made since February 2006 to an emergency response centre in Paisley were stored on a portable hard drive being transported by courier TNT.
The hard drive was sent on June 9 to MIS Emergency Services in Manchester, which runs the ambulance service's computer system, but did not arrive.
"All security procedures for the transfer of data were followed, however TNT have advised that they cannot find the package," said Scottish Ambulance Service Acting Chief Executive Pauline Moore.
"The information stored on the disk is fully encrypted and it would be extremely difficult to access any names or addresses."
Data on the drive included addresses of incidents, some phone numbers and some patient names.
It is the latest loss of confidential data since HM Revenue and Customs lost in the post child benefit records of 25 million people last October.
An Independent Police Complaints Commission report into the HMRC data loss is due to be published on Wednesday.
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