Madhouse was adapted from the 1969 novel Devilday by Angus Hall. A horror and crime writer, Hall had written the novelization for Amicus rival Hammer Studios' Scars of Dracula (1970).

Madhouse had been intended originally as a vehicle for Vincent Price and The Abominable Dr. Phibes director Robert Fuest and was announced in the trades as such as early as April of 1970.

During pre-production, Madhouse was known as Devilday and throughout principal photography as The Revenge of Dr. Death.

In an interview published in 2000, Robert Quarry maintained that his dialogue was so bad that he rewrote it and that Vincent Price asked Quarry to rewrite his dialogue, too.

Interior scenes of Herbert Flay's country home were filmed at Surrey's Pyrford Manor, a 15th century residence often used by Queen Elizabeth I.

The recording of "When Day Is Done" sung over the end credits is performed by Vincent Price and was removed from video cassettes of Madhouse due to copyright issues.

During the editing of Madhouse, producer Milton Subotsky ordered drastic cuts that greatly affected dialogue exchanges, prompting a letter of protest from director Jim Clark to star Vincent Price decrying Subotsky's "pure butchery."

Research compiled by Richard Harland Smith

Sources:
The Horror People by John Brosnan
Vincent Price: A Daughter's Biography by Victoria Price
Peter Cushing: An Autobiography and Past Forgetting
IMDB