Tatum O'Neal trained with a professional sports trainer for several weeks before filming began in August 1975, so she could get her pitch "pitch perfect", so to speak. Although some of the pitches in the movie were done by stunt doubles, O'Neal did the bulk of them on her own. (Rare for a movie like this,)
The Bears were the only team in the league whose nickname and colors were not borrowed from a major league baseball team. This helped reinforce their status as misfits and underdogs.
Jodie Foster was cast at one point as Amanda Whurlitzer but dropped out in order to play Iris in Martin Scorsese's Taxi Driver (1976).
In the film, neither of the Aguilar brothers could speak any English. In reality, according to an interview in a 2001 Sports Illustrated "Where Are They Now" issue, Jaime Escobedo (Jose) and George Gonzales (Miguel) were in fact fluent in English, and didn't know how to speak in Spanish. They had to learn their entire dialogue strictly in Spanish before filming began.