An episode featuring a Chicago cop taking a bribe led to Mayor Richard J. Daley banning any TV show or film from doing any shooting in Chicago at all. That ban was later broken with the filming of The Blues Brothers (1980).
The show was inspired by the exploits of legendary Chicago crime fighter Det. Supt. Joseph Morris. Det. Lt. Frank Pape, who worked for Morris, served as a technical adviser on the show, without getting credit, however, because of his active status within the Chicago Police Department, which banned police moonlighting.