The film premiered at the Lyric Theatre in New York City as a second feature with Journey to the Far Side of the Sun (1969).
Final film of Chick Chandler.
Although unrelated to Mario Bava's earlier Italian film La Ragazza che sapeva troppo (the English title of which is also "The Girl Who Knew Too Much"), Lyon's film pays homage to Bava's in several important respects. The chief cast of the Italian film play bit parts or cameos in the US film - Letícia Román is a flower-seller in the underground scene, and actor Dante DiPaolo appears as Dr. Hims. Neither has a speaking part, although Roberto Calvi (who plays a tramp) speaks English briefly.
Regarded by many film historians as the first giallo.
Final film of actress Laurie Mitchell .