Trivia

The film was predicted to be a flop, was shelved by Paramount and eventually released in only one theater in New York. Screenwriter Ben Hecht demanded that his name was taken off the credits. As a result of strong word-of-mouth the movie became an enormous hit and won Hecht the first of his two Academy Awards.

"Underworld" was submitted to the British Board of Film Censors three times:

• The first submission, under the original title, was rejected because of the sympathetic portrayal of a criminal.
• The second submission, under the title "The Penalty" and with cuts, was rejected.
• The third submission, under the title "Paying the Penalty" to give the movie a more moral stance, and with additional cuts, was passed with an "A" certificate.

The gangster Buck Mulligan (named after a character in James Joyce's Ulysses) was based on the Chicago gangster Dion O'Banion. O'Banion also had a florist business and was rubbed out in his flower shop.

Generally considered to be the very first Film-Noir feature film.

This film won the Best Writing, Original Story Oscar for its only Academy Award nomination.