The Alliance party leader has announced that she is due to undergo surgery for endometriosis.
Writing on social media, Naomi Long revealed that she has suffered from the condition for most of her adult life.
She expects to be off work for up to 10 weeks to recover.
The party has said an interim leader will not be appointed during Ms Long’s absence.
Ms Long wrote: “This isn’t something I talk about much, but I wanted to let you know what’s happening.
“I suffer from very aggressive endometriosis and have done so most of my adult life.
“However, the chronic pain and other symptoms of the condition have become increasingly severe and hard to manage over the last few years.”
What is endometriosis?
Endometriosis is the name given to the condition where cells like the ones in the lining of the womb (uterus) are found elsewhere in the body – including the bowel, ovaries or bladder.
The cells act in the same way as during a period, building up and breaking down.
But they have no way of getting out of the body and can cause inflammation, pain and the formation of scar tissue.
“I’ve decided that surgery is now the best option,” she added.
Naomi Long said that she had “a couple of emergency admissions to hospital” as a result of the condition.
“I was originally due to be admitted in January – however, due to the collapse of the Assembly and elections, I postponed until April.
“Then the General Election was called, and I had to postpone again for that campaign and the talks process.”
Get Quotes on Home Insurance
Ms Long said she will also be taking a break from social media while she is “recuperating”.