Originally, the film was intended for Ryan O'Neal and his son, Griffin, but he would instead appear in another 1979 boxing film, The Main Event.
Because the production didn't have an actor to play the lead title role, shooting was delayed from February to March of 1979.
After Ryan O'Neal withdrew from the film, Robert Redford was approached to play the main character, but after he wanted script changes that would've pushed the beginning of the film's principal photography, the production looked for someone else.
Jon Voight had a physically tough time working on the film. In order to play the lead character, he had to learn things quickly and underwent an intensive training schedule. Voight was trained in boxing by Jimmy Gambina (who did the fight sequences for Rocky).
For the role of T. J. Flynn, two thousand kids were interviewed and Ricky Schroeder was screen-tested for the part. Originally, the producers offered Schroeder the role on the condition that his parents sign a seven-year exclusive contract on his behalf. At first, Schroeder's mother turned down the offer, but later, the producers offered Schroeder the role with no strings attached.