The presence in the Mariana Islands of certain chronic neurologic diseases among the Chamorros has been known for several decades. Systematic studies from 1952 have confirmed the high prevalence of amyotrophic lateral sclerosis and parkinsonism dementia in this area. These disorders are 50 times more common among the Chamorros in the Marianas than elsewhere in the world. Amyotrophic lateral sclerosis and parkinsonism dementia complex in the Chamorros appear to be different expressions of a single highly prevalent disease process. The clinical syndromes represent a neurologic disorder with varying combinations of dementia and involvement of the upper and lower motor neurons and the extrapyramidal system. Amyotrophic lateral sclerosis is also seen within the immigrant Carolinian community of Saipan, in a slightly different form. The symptoms and signs, which are mostly referable to the spinal cord, remain slight for over several years. A few cases of amyotrophic lateral sclerosis and parkinsonism dementia have also been noted among Filipinos and whites. The presence of these syndromes among other than the Chamorros raises serious questions with regard to genetic transmission alone and calls for a most energetic search for a possible exogenous agent. No such exogenous agents have yet been found. It is possible that the background may be an inborn biochemical error at the enzymatic level and that the exogenous factor may be a virus.