Sol Yurick wrote the original book as a rebuttal to the romanticized view of street gangs presented in West Side Story (1961) based on his experience as a New York City welfare department worker.
David Patrick Kelly improvised Luther's "come out to play-ay" taunt, basing it on an intimidating neighbour of his.
The original poster featured the words "These are the armies of the night. They are 100,000 strong. They outnumber the cops five to one. They could run New York City." This upset and outraged many people; some tried to have the film banned.
Vermin was scripted to be killed by the Lizzies, but Terry Michos, who played Vermin, made his character more comical and thus more memorable. The ploy worked, and the death scene was taken out.
In the subway where Mercy is running with Fox's double, she fell and broke her wrist because the actor didn't let go of her hand. This is why she later appears in a jacket as it is covering up the cast. The filmmakers had Mercy disappear from the film for a while, meeting up with the Warriors at the subway platform and saying that she stole the jacket she was wearing before arriving there.