Soon after filming was completed, Fiona Lewis (who was then getting quite large supporting roles in British films) gave an interview to a newspaper in which she said that the filming had been a nightmare, and describing director Jerry Lewis as the biggest egomaniac she had ever met. Interestingly, when this movie opened in Britain, several months after its American opening, Ms. Lewis was nowhere to be seen in it.
This is the only film in which Peter Cushing appeared as Frankenstein with Ygor as his assistant and the only film in which he appeared as Frankenstein with Christopher Lee playing a vampire.
This is one of four films released in 1970 in which Christopher Lee played Count Dracula. The others are Count Dracula (1970), Taste the Blood of Dracula (1970) and Scars of Dracula (1970).
In the ballroom scene, when the band leader makes his announcement, the actor's voice is dubbed over with Jerry Lewis' voice.