Trivia

The odd symbol that appears again and again - on Wilbur's ring, on his grandfather's staff, in the design on the main floor, etc. - is an ancient Native American symbol commonly termed "Thunderbird in sun".

"The Dunwich Horror" was one of the last on camera acting appearances by Ed Begley. This underrated character actor passed away in 1970, the year of the film's release.

Two sound-stages were used for Sandra Dee's nude scene to prevent "peeping toms." One sound-stage was used to shoot the scene and the other was used to house the cast not needed in the shot.

AIP had previously wanted Mario Bava to direct an earlier, proposed version of this film, titled "Scarlet Friday", that would have starred Boris Karloff (with whom Bava had made I tre volti della paura (1963)) and Christopher Lee (who had been the star of Bava's Hércules al centro de la tierra (1961) and El cuerpo y el látigo (1963)). The project was taken away from the director after the disastrous US theatrical run of Dr. Goldfoot contra el mundo (1966).

In a Psychotronic magazine interview (#21 - 1995) Dean Stockwell stated: 'I happened to be a fan of H. P. Lovecraft, and it was nothing like H. P. Lovecraft, so that was a little disappointing. Kinda sophomoric, not what you really call a horror film, whereas Lovecraft's writing can be scary at times.'