In the past decade attention has been focused upon the Mariana Islands of the western Pacific as an area of particular interest to neurologists and epidemiologists. The prevalence of amyotrophic lateral sclerosis among the indigenous Chamorro population is unusually high.1-4Recent reports have described the clinical,5pathological,6and electroencephalographical7features of another endemic neurological disease of Guam, "the parkinsonism-dementia complex." It is the purpose of this communication to describe the epidemiological characteristics of this latter disorder. The Marianas Group and Its People The Marianas form a chain of 15 volcanic and coral islands between 13 and 21 degrees N. latitude at 145 degrees E. longitude (Fig. 1). Guam, a territory of the United States, is the largest and most southern of the group. It is located 5,100 miles west-southwest of San Francisco and 1,600 miles south of Tokyo. It lies within the Tropic of Cancer, and