The plans for a sequel began before this film was even in theaters.

Actor Ken Jeong improvised a nudity in which he jumps out of a car trunk naked. Originally Jeong's character was supposed to have clothes on, but he suggested that his character be naked instead.

According to Bradley Cooper, "the people in Las Vegas never even noticed [a] movie was being filmed." He had tiger scratches on his neck, all bloodied, and Justin Bartha was beet red, and nobody even stopped to double-take.

Director Todd Phillips waived his usual salary and received percentage of the ownership of the film instead. On Howard Stern, Phillips said that due to the film's box-office success "this is [his] Star Wars," referring to Alec Guinness who knew that Star Wars would be a box-office success and received 2% of all of the gross earnings (Star Wars made $798MM in theaters alone, nor to mention DVD and merchandising).

The Hangover was Bradley Cooper and Justin Bartha's third film together. The other two were Failure to Launch and New York, I Love You.