Peter Cushing dresses as Dracula, an in-joke reference: he frequently played Van Helsing in Hammer's Dracula series.
Final film Price made for AIP after a collaboration that began 14 years earlier with "House of Usher."
During the costume party, Robert Quarry is dressed in his outfit from the film "Count Yorga, Vampire" (in which he played the title role).
During the last scene and over the closing credits, it is Vincent Price whose singing voice we hear.
The title credits include special participation by Basil Rathbone and Boris Karloff. Both men were deceased, having died in the late 60s. However, they appear in clips from real films they acted in with Vincent Price: Rathbone in Tales of Terror and Karloff in The Raven.
The Dr. Death movie clip played at the beginning of Madhouse is from the end of The Haunted Palace (1963). The voices have been dubbed to fit with Dr. Death.
The film scenes shown at the costume party at Olivers house as a tribute to Paul (played by Vincent Price) is an actual Vincent Price film, The Pit and the Pendulum.