A computer containing over 171,000 confidential blood donor records and other files from the Irish Blood Transfusion Service has been stolen.
The data, which the Blood Service says was securely encrypted, was given to the New York Blood centre in December on a computer disk.
It was part of a software upgrading programme for the Irish Service.
The laptop with the disk was stolen on 7 February when a member of the New York Blood Centre was mugged outside his home.
The Irish Blood Transfusion Service says it is deeply concerned at the theft and was first notified of the incident on 8 February.
It plans to issue letters later this week to each of the 171,324 donors affected to reassure them and advise them of the possibility, however remote, that their personal data might be accessed.
It also intends to contact another 3,294 patients whose hospital blood records are also involved.
An information helpline that donors can contact will operate from 8am tomorrow morning. The number is 1850 731 137.