Gaten Matarazzo is undergoing a "big" operation for cleidocranial dysplasia (CCD).
The 'Stranger Things' actor - who plays Dustin Henderson in Netflix's sci-fi horror series - is set to have his fourth surgery to treat the birth defect, which affects growth of the bones and teeth.
Although the 17-year-old star didn't reveal the full nature of the procedure, he wrote on Instagram: "Surgery number 4!
"This is a big one! To learn more about Cleidocranial Dysplasia and how you can help those with the condition you can go to ccdsmiles.org."
Gaten's post was flooded with well wishes from fans and co-stars, including Millie Bobby Brown.
She commented: "Good luck love!! sending my love gate (sic)"
Matty Cardarpole added: "You're super brave and strong man. Good vibes to you."
Gaten has been open about his experience with CDC in the past, and even insisted his character Dustin also had the same condition.
In 2016, he explained he felt it was importance to "raise awareness for it" and help people feel confident.
He said at the time: "It affects your facial growth, your skull growth. It affects your teeth, so that's why I don't have any.
"I feel like putting it into the show is really raising awareness for it. I just want to raise awareness for it and let people know that it's not something they should be afraid of showing."
Meanwhile, last year Gaten admitted the 'Stranger Things' cast would be happy with the show ending when there's "no more story" left to tell.
He said: "I think we should stop whenever the story feels like it needs to be completed.
"I don't think any season should be pushed. If there's no more story, there's no more story."
While his castmate Noah Schnapp said five seasons would be ideal, Finn Wolfhard isn't sure the show needs that many.
He added: "I think we definitely need one more to tie it all up.... Either one or two more, for sure. At least one more to tie it up."