Director Dario Argento's original idea was that the ballet school would accommodate young girls no older than 12 years. However, the studio and producer Salvatore Argento (his father) denied his request because a film this violent involving children would almost certainly be banned. Dario raised the age limit of the girls to 20 years but did not rewrite the script, hence the naiveté of the characters and the occasionally childlike dialogue. He also put all the doorknobs at about the same height as the actress' heads so they would have to raise their arms in order to open the doors, just like children.
The woman playing Helena Markos was not credited. According to Jessica Harper, she was a 90-year-old former prostitute who director Dario Argento found on the streets of Rome, Italy.
Director Dario Argento had cinematographer Luciano Tovoli watch Snow White and the Seven Dwarfs (1937) to have him model the color scheme of that film for this one.
Director Dario Argento composed the creepy music with the band Goblin and played it at full blast on set to unnerve the actors and elicit a truly scared performance.
For the wide shots of the "maggots" falling from the ceiling, the crew would drop grains of rice down onto the actresses from above.