There were no special effects used to deepen Kevin Grevioux's (Raze's) voice. His natural speaking voice really is that deep.

Days before its U.S. opening, the president of Screen Gems greenlit not only a sequel to the movie Underworld but also a prequel after seeing the number of tickets sold in Canada alone.

Not only in this movie did Kate Beckingsale meet her future husband Director Len Wismen, but at the time she was in a long term relationship with actor Michael Sheen, who played Lucian. Michael and Kate share a daughter together who appears in the film during a flashback of when Selene was younger. Michael and Len have since become very close friends and have collaborated with each other on Rise of the Lycans. Michael and Kate still remain very close and both are raising their daughter equally. Kate has stated that Micheal is her best friend and that they still remain very close.

The 'attack dogs' outside of the vampire mansion were actually very docile and playful canines, which were the only dogs available at the time. For the scene where they chase Scott Speedman, director Len Wiseman had to film short clips of the dogs running and later put in sounds of vicious barking. If you look closely though you can see their tails happily wagging back and forth.

Rhona Mitra, who was considered for Selene, would later play Sonja in Underworld: Rise of the Lycans.

Kate Beckinsale first met husband Len Wiseman on the set.

Scott Speedman suffered a concussion when a set prop that was supposed to be a piece of the wall Viktor had just thrown him through hit him in the head.

According to writer/actor Kevin Grevioux, the model for the Selene character was the psychic vampire of the Hellfire Club in the X-Men comics, the Black Queen Selene.

A kiss between Michael and Selene (when she unchains him in Lucian's lab) appears in the trailer but was not included in the theatrical release or on the DVD, but it is on the unrated version of Underworld.

'Selene', the heroine's name, is the Moon in Greek mythology.

Although Lucian is the lead Lycan, he only ever shifts once into his full werewolf form, and that's in a flashback scene.

The movie was initially pitched as "Romeo and Juliet for vampires and werewolves."

Len Wiseman opted to shoot many of the action and effects sequences live, without computer imagery. In one scene, for instance, a werewolf seems to run more than 50 mph behind a speeding car. This was shot using an elaborate rig towed behind a vehicle, with actor Michael Sheen doing the closeups of the running and the jump onto the car, while the long shot of Lucian running was done by stuntman Todd Schneider.

The name Corvin, derived from Corvinus is most likely a reference to Mathius Corvinus of Moldavia who was an ally of Vlad Tepes, the inspiration for Bram Stoker's Dracula.

White Wolf, Inc. and author Nancy A. Collins filed suit against Sony Pictures, Screen Gems, and Lakeshore Entertainment on 4 September 2003 for copyright infringement little more than a week before the theatrical release of Underworld, alleging 17 counts of copyright infringement, and claiming over seventy points of unique similarity between White Wolf's role-playing games, "Vampire: The Masquerade", "Werewolf: The Apocalypse", and their creation, "The World of Darkness", in which the games are set. Nancy A. Collins is the author of a short story, "The Love of Monsters", published in 1994 by White Wolf Inc, and set in the World of Darkness, which she claims the entire plot of Underworld is based on. The suit was settled out of court, for an undisclosed amount.

The woman in the picture Selene finds in Michael's apartment is the wife of writer Danny McBride.

The underground tunnels used for the Lycan/vampire battle at the end of the movie were mostly shots of the same small rubble set, but shot from different angles and lighting in order to make it look like different locations underground. During the DVD commentary, director Len Wiseman laughed at this fact and pointed out the scenes in which this trick photography was used.

This movie marked Len Wiseman's feature film directorial debut.

In the newly released 2017 4k remaster of Underworld, the scene where Michael recounts the events of Sonya's demise when he speaks with Lucian in the film is replaced with the actual scene from Underworld: Rise of the Lycans instead of the original flashbacks used in the 2003 version.

The name of the city where the film took place is never mentioned. However Michael's address that we can see on Selene's computer is an apparently misspelled Hungarian address (it should be "Lakatos József u. 39." while it is spelled as "Laktos Joszef 39 ut." instead). You can also read "Eötvös utca", a street in the historical downtown of Budapest, many times while driving by to the secret interrogation place and there is an ad in the subway station where the shooting takes place in the beginning of the movie referring to Szentendre which is a small town near Budapest (obviously left there as non-Hungarian viewers don't understand the text anyway).

Michael Sheen and Wentworth Miller were both interviewed for the Scott Speedman role.

Len Wiseman wanted David Bowie to appear as a vampire at the vampire's feast, but due to some time problems it did not work out.

Halle Berry, Milla Jovovich and Rhona Mitra were considered for the lead role.

The actors only had two weeks of wirework and prop training for the movie.

The ancient tome which Selene uses to research the history of Kraven features pages from three different sources, all shown before "The Fall of Lucian." The first, featured on the page with the seals of Amelia, Viktor, and Marcus, is an ancient Hungarian text entitled Funeral Sermon and Prayer. The second is an old Catholic Roman Missal, including the Propers of the Second Mass for All Souls' Day. The third is a book entitled Instructissima Bibliotheca Manualis Concionatoria, which is also featured between pages of the story of the fall of Lucian.

Kate Beckinsale's character Selene is not seen speaking until roughly 12 minutes into the film, however her narration starts almost as soon as the movie does.

It only took two days to film Wentworth Miller's role

When Selene first encounters Lucien, she gets cornered by Lycans and fires her guns through the floor in order to escape. Eleven years prior, this cinematic stunt was performed by the main character in the 1992 film "Nemesis."

After the underground station fight, the tunnels where Selene kills a Lycan are a champagne cellar.

Selene is a Greek name. Seleneophile is a name for someone who loves the moon and Selene is the Greek name for the Goddess of the Moon.

The song used for the trailer is called "Red Tape" by 'Agent Provocateur'.

Although many weapons are used in this film, the iconic handguns in the subway scene are the Beretta 92FS, heavily modified to look like the Beretta 93R. They have also been modified to fire in full automatic mode (3-Shot Burst, actually; If you slow down the scenes you can see 3 rounds fire per trigger pull) and contain barrel weights. They use 20 or 30 round box magazines.

Michael sheen plays a Lycan (aka) a werewolf who dislikes vampires because of the war in which his vampire girlfriend was killed in a later movie however he plays a vampire who hates werewolves in the Twilight Saga.

"Underworld" was lightly parodied as "The Wreathing" in the 2010 BBC radio/audiobook adaptation of the Simon Brett's "Cast, in Order of Disappearance". The original story (written in 1975) was relocated to the set of the vampire film for the audiobook, and starred Bill Nighy ("Viktor" in the Underworld films) as Charles Paris, a minor British actor and amateur sleuth, engaged to play the role of "Szabec", a middle management vampire in an organised vampire society. The mystery is set around the actors while on and off set for the film. Martine McCutcheon plays a character which is loosely modelled on Kate Beckinsale.

The end credits song named "Coward" is wrongly credited as performed by Renholder while it actually is a song from Black Light Burns.

During the flash back scenes it is shown that Sonja has blonde hair, but in the movie Underworld rise of the lycans she is shown with black hair.

Body Count: 67.