In view of th reported high prevalence of otitis media and mastoiditis in the present-day Indian inhabitants or Arizona, when an opportunity arose to examine the skulls of prehistoric Indians in the collection of the Arizona State Museum, University of Arizona, Tucson, Arizona, it was thought that determining the prevalence of mastoiditis in them might be helpful in the treatment and prevention of mastoid infections in the present-day population. Our findings are compared with those of two other studies of mastoiditis in prehistoric Indians in another area of the United States.