A prospective study of the incidence of new cases of amyotrophic lateral sclerosis and parkinsonism-dementia was conducted by follow-up of 899 Chamorros in the Mariana Islands who participated in a baseline medical examination and interview in 1968. During a 15-year follow-up period, 28 new neurologically confirmed cases occurred, including 23 with parkinsonism-dementia and five with amyotrophic lateral sclerosis. The low incidence of amyotrophic lateral sclerosis was consistent with the reported disappearance of this disease from the Mariana Islands. However, the continuing high incidence of parkinsonism-dementia was not in agreement with a similarly reported disappearance nor with the hypothesis of a common cause of both diseases. Of 23 selected baseline examination variables, only the preference for traditional Chamorro food was significantly associated with an increased risk of parkinsonism-dementia. A review of the accumulated epidemiologic data on these neurologic diseases in the Mariana Islands did not support the current hypothesis that a chronic nutritional deficiency of calcium is a cause of both amyotrophic lateral sclerosis and parkinsonism-dementia.