During production, none of the cast was told who the killer was, and because of the high amount of red herrings in the film, many of the actresses were convinced it was someone else doing all the murders.

The film was highly censored in English language countries. Because of this, parts of the English dub do not exist for the film. On uncut DVDs, these scenes are played with their original Italian language track and English subtitles.

Suzy Kendall, who was British, was dubbed by American voice artist Susan Spafford.

Quentin Tarantino showed his print of the film at the 1999 QT-Fest. His fellow filmmaker Eli Roth has cited the film among his favorite Gialli and an influence on Grindhouse (Tarantino, 2007) and Hostel: Part II (Roth, 2007).

The fuzztone guitar instrumental featured in the American theatrical trailer is a song called "Hippy" by Alan Parker.

An alternate ending was shot with the killer surviving in the end.

The number of victims in the film is 9.