The number of patients who waited more than 12 hours at A&E units in Northern Ireland over Christmas almost tripled compared to the same period last year.
A total of 928 patients waited longer than the 12-hour target waiting time at hospital emergency departments between 24 December 2017 and 1 January 2018.
By contrast, 348 waited more than 12 hours in the 2016-17 holiday period.
The figures also show an 18-fold increase in those from Christmas 2015-16.
During that seasonal period, only 48 patients were kept waiting longer than the 12-hour ministerial target.
The figures were obtained by BBC News NI from Northern Ireland's Health and Social Care Board (HSC).
The board said emergency departments across the UK and Ireland were all facing significant pressures.
There was a year-on-year rise in complex and serious conditions, particularly amongst the growing frail and elderly population, said the board in a statement.
This is on top of an already very busy system, that makes responding to spikes in pressures more difficult, it added.
All of the pressures facing the health and social care service in Northern Ireland emphasised the need for widespread reform of the current system, said the board.
The Christmas and New Year holiday period was exceptionally busy for all of Northern Ireland's hospital emergency departments (EDs), with an unprecedented number of patients seeking treatment.
An HSC spokeswoman confirmed there had been no change in the methodology for compiling the waiting time statistics.
The current ministerial target states that no-one should wait longer than 12 hours to be treated, discharged or admitted.
On Thursday, HSC deputy chief executive Michael Bloomfield said: "Over this holiday period there has been a particular increase in acuity and illness of people coming into hospital, that has presented significant challenges."
He added that patients with "less urgent conditions who should more appropriately see their GP or indeed a pharmacist", were adding to the problems in EDs.
The HSC urged the public to only attend for urgent and life-threatening conditions.