The theme song was originally composed by Ennio Morricone as a lullaby. Sergio Leone insisted that he wanted the "deguello" trumpet dirge - played by Mexican troops before a battle to signify to the enemy that there will be no quarter given - that was used in Rio Bravo (1959), believing it was a "public domain" piece. Finally, he settled for a "Mexican trumpet" arrangement of the original Morricone piece.
According to Mickey Knox, Sergio Leone ran Yojimbo (1961) in a Moviola while constructing the script and blocking the action.
The trailers and promo material in the United States list the title of this film as "A Fistful of Dollars". The on-screen title on the film itself gives the title as simply "Fistful of Dollars".
The film had its North American premiere at the Carlton Theatre in Toronto, beginning its run on December 23, 1966.
The trivia item below may give away important plot points.
Mario Brega appears in all 3 of the Dollars Trilogy movies, and in all 3 movies, his character meets an unfortunate demise. In this movie, his character of Chico is crushed by a runaway large barrel pushed by Joe (Clint Eastwood). This is the only movie in the Dollars Trilogy where Eastwood's character was directly involved with Brega's character's demise.