Alliance Party leader Naomi Long is to take a 10-week break for major surgery.
The East Belfast Assembly member said she had coped with endometriosis for most of her adult life.
Endometriosis happens when cells like the ones in the uterus lining spread to elsewhere in the body. It can cause chronic pain due to inflammation.
Ms Long wrote on Facebook: "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."
She said she expected to be off work for up to 10 weeks recovering from an operation in the coming days. She said the procedure had originally been scheduled for January but was postponed due to the collapse of the Assembly followed by snap elections.