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.
In one take, Michael Beck (Swan) swings a bat into Deborah Van Valkenburgh's face (in the scene where he throws it at the cop). She was rushed to the hospital at 3 am for stitches and still has a scar.
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.