A&E waiting times: Rise in number waiting more than 12 hours
More patients had to wait over 12 hours for treatment at seven of Northern Ireland's A&E departments in January.
Department of Health figures show that 380 people waited more than 12 hours, compared to 92 in the previous month.
In January, 54,910 people went to emergency departments compared to 56,656 in December.
The Ulster Hospital had the biggest rise in those waiting longer than 12 hours and Antrim Area Hospital had the biggest fall.
There was also a fall in the percentage of people seen and treated within four hours, from 73.5% in December to 71.4% in January. The target is 95%.
Figures released in December showed that Northern Ireland was the worst-performing region in the UK for seeing patients in emergency departments within the four-hour target.
The Health and Social Care Board said it had been a challenging month, with significant pressures during the first week of January coming after a two-week period of substantially increased demand.
Chief executive Valerie Watts said delays were regrettable, but "it must be noted that significantly fewer people are waiting over 12 hours in emergency departments, compared to five years ago".
"In 2011/12, over 10,000 patients waited longer than 12 hours in our emergency departments - that had almost halved in 2012-13 to 5,500 and was just over 3,000 last year," she said.
Ms Watts said the board had been working closely with trusts to alleviate pressures in emergency departments and throughout the hospital system.