The filmmakers were unable to get permission from the South Korean government to film in the actual hospital, so scenes in the film set in Gonjiam were actually filmed in the National Maritime High School in Busan, with the production team adhering closely to the floor plan of the hospital to recreate exactly the same exterior and hallways.
Just shortly before the theatrical release, the owner of the asylum filed a lawsuit against the film being shown in theaters, claiming that the film will have negative effects on the sale of the building. However, a Seoul court in late March 2018 ruled in favor of the film being shown.
The film, a critical and financial success, became the third most watched horror film in South Korea after Phone (2002) and A Tale of Two Sisters (2003).
Conception for the film was inspired by a 2012 article, where CNN Travel selected the abandoned Gonjiam psychiatric facility as one of "7 freakiest places on the planet."
In the ending scenes of the film, the Horror Times live stream website is visible. Examining the website closely shows that the live stream has 1234 plays, with 56 likes, 78 shares, and 910 comments. In order, these numbers make the sequence 12345678910.