Loosely based on the house party of Corey Worthington (Delaney) in Australia. The then-teenager posted the address of his house party on MySpace, attracting around 500 people and caused over $20,000 in property damage. His party attracted journalists and was widely disliked by the public, having been assaulted by a group of teenagers after the events.
Some of the footage was shot by the cast with cell phones, Flip, iPhone and Blackberry phones that were handed out by the crew. Over 10 hours of footage was recorded mostly due to the cast continuously recording.
Filmed on a set as opposed to a real neighborhood in order to minimize disruption to the locals. This proved to be a wise move as the set was effectively trashed during production. Nevertheless, nearby locals complained to Burbank police about the noise.
When the camera catches the young lady urinating, she gestures for it to shoo away. According to a popular Internet rumor, the lady was urinating in a corner due to a lack of bathrooms on the set, and was irritated by this violation of her privacy. Only by paying her an extra amount of money were the editors allowed to use the footage in the film. However, her expression is ambiguous, and she almost appears to be smiling or laughing while gesturing the camera away from her body function.
Director Nima Nourizadeh's background in commercials landed him the job as helmer. He initially moved from London to Los Angeles for what he thought would be a two-week stint on refining the screenplay. He ended up staying for two years.
The studio disliked the title "Project X", feeling it was more appropriate for a science fiction film, and suggested re-titling the film "Happy Birthday, Thomas Kub", after a message seen on the school jumbotron. Producer Todd Phillips countered this by saying that firstly the studio's suggested title made the film sound like a good-natured buddy movie instead of the no-holds-barred comedy it actually was, and secondly that he and the writers saw Costa as more the focal character than Thomas. In the end, neither party could think of a better title, and so the finished film stuck with the "Project X" title.