The event described in the beginning of the film is based on an actual event. In February 2000 at a US military facility located in the center of Seoul, a US military civilian employee named Mr. McFarland was ordered to dispose of formaldehyde by dumping it into the sewer system that led to the Han River, despite the objection of a South Korean subordinate. The government attempted to prosecute Mr. McFarland in court, but the US military refused to hand over the custody of Mr. McFarland to the South Korean legal system. Later, a South Korean judge convicted Mr. McFarland in absentia. The Public was enraged at the government's inability to enforce its law on its own soil. In 2005, nearly five years after the original incident, Mr. McFarland was finally found guilty in a court in his presence. However, he never served the actual prison sentence, and there have been no sightings of a mutant creature in the Han River - yet.
As of March 2009, this is the highest grossing film to date in South Korea, selling a total of 13,019,740 tickets. This means that over 20% of the South Korean population watched the movie (although there were also some who went to see it several times).
Joon-ho Bong was impressed with Scott Wilson after having seen him in, among other films, Monster (2003). He approached the actor by sending him a copy of the script and a DVD of his previous film, Memories of Murder (Memories of Murder (2003)). The actor later agreed to be in the film.