A review is to be carried out after the third data breach at NHS Orkney in the space of less than two months.
The new incident saw a discharge letter being given to the wrong patient.
It follows dozens of Covid-19 test results being sent to a local business by mistake, and confidential personal details relating to staff travel sent to a journalist.
New interim chief executive Michael Dickson has requested an "external rapid review" of procedures.
In a statement, Mr Dickson apologised and said it was "not acceptable" for such things to be happening.
He said: "It is with disappointment that I have been alerted, once again, to another data breach within NHS Orkney. The circumstances of the incident are currently being investigated so we can understand how this occurred.
"The recent run of data handling incidents indicates to me there is a problem within NHS Orkney that extends beyond minor errors that are quickly rectified.
"I have requested an external rapid review of the board's standards, policies, procedures, training and leadership in this area."
He added: "I understand why the people of Orkney will feel angry and concerned by this latest breach. I want to reassure them that steps are being taken to identify why these mistakes continue to happen and that corrective action is being taken.
"To the patients involved, I apologise. There is no excuse for this and it will be dealt with."
Mr Dickson - who currently leads the health board in Shetland - is also heading Orkney for the time being.
On 30 June it was announced NHS Orkney's chief executive designate Iain Stewart, who came under fire for leaving the islands for his mainland home during lockdown, would no longer take up what should have been his new job.
Mr Stewart said he had only travelled to his Black Isle home when necessary.