The director of 'Harold and Kumar' also directed 'Dude, Where's My Car?'. This is given a nod near the end of the film where the guys meet up with Neil Patrick Harris again, Harold says "Dude where's my car?"

When Harold is cut off by the orange bronco, the man inside screams "better luck tomorrow," a movie John Cho starred in 2002.

Neil Patrick Harris's part was originally written without him knowing about it. After the movie was being cast he was approached and agreed to do it.

Kal Penn, who plays Kumar, is allergic to nuts. During the scene where he crashes through the ceiling at the station, a nut-based mixture was used for the dust that comes out of the heat vent. Upon entering the set that day, Penn had a severe allergic reaction, and had to be taken to a hospital. Later, a safer, nut-free concoction was used for the scene.

It was mentioned a few times during the movie that "Sixteen Candles" (1982) is one of Harold's favourite movies, but in real life, John Cho (Harold) really hates that movie. He was traumatised when he first saw it because of the humiliating portrayal of the Asian character (Long Duck Dong).

Kal Penn, who plays Kumar, is a vegetarian, so special veggie Castles had to be made for him.

When the character who plays Kumar went to the audition (in real life), the first thing they said to him when they saw him was "Where's your turban?"