Often considered Mario Bava's most influential film and the film that started the "slasher" craze, which is still popular. Many fans of the genre consider this the grandfather of the modern slasher film.
Dario Argento loved the film so much, he had a friend (who was a projectionist) steal him a print of the film during its first run in Italy. The theater ended up showing Blood Brides (1970) to replace the stolen print for the remainder of the film's run there (about a week and a half according to Argento. He possesses the print to this day).
The shooting locations didn't have woods, so Mario Bava created them by setting up tree branches to pass through the camera shots. According to Laura Betti, the trickery would look so silly during shooting that the cast and crew would often laugh hysterically.
One of the re-release titles for this film was "The Last House on the Left Part II", even though this film has nothing to do with Wes Craven's The Last House on the Left (1972). In fact it was made a year earlier than "Last House".
Federico Fellini once commented that he worked on writing a horror film for an acquaintance who gave him a script with numerous depictions of murders, but not one thread of story connecting them. Many believe it was Mario Bava he was referring to, in particular this movie. After a month of having all his ideas rejected, he told the filmmaker to shoot the script and figure out the story later. He stated this might have been the best advice he ever gave another filmmaker.
Due to the film's low budget, most of the locations in the film belonged to director Mario Bava or members of the crew. The interiors of Countess Federica's home was shot at a favorite villa of Bava's and the interiors of Frank Ventura's country house were shot at a summer home owned by the producer.
The 1971 Avoriaz Film Festival jurors awarded the film the Best Makeup and Special Effects Award. Carlo Rambaldi's effects work also earned the film a "Special Mention" Award at the prestigious Sitges Festival in 1971.
One promotional gimmick was that every ticket holder was required to pass through The Final Warning Station, where a theater worker warned you face to face that this may be the last 'shock' film you will ever want to see.
The film not only had numerous release titles, but also several working titles throughout the production. Among them "The Stench of Flesh", "Thus Do We Live To Be Evil" and "That Will Teach Them To Be Bad". "Reazione a catena" ("Chain Reaction") was the final choice.
The origin of the film was Mario Bava's desire to work with Laura Betti again. Betti had previously appeared in Bava's Blood Brides (1970) and the two had gotten along so well they concocted the premise of this film for another project together.