Amyotrophic lateral sclerosis (ALS) and parkinsonism-dementia (PD) occur in the highest recorded incidence among primitive Auyu and Jakai people on the southern coastal plain of West New Guinea, in association with a heretofore unrecognized subacute, often recurrent, paralytic "poliomyeloradiculitis" (PMR). Ninety-seven cases of ALS, 19 cases of PD and 18 cases of PMR were recorded, with mean ages of onset of 33, 43, and 26, respectively, in a small affected population of only about 7000. The ecology, culture, and diet of the remote, primitive ALS- and PD-affected people are indistinguishable from that of their unaffected neighbors, except for a remarkable deficiency of calcium and magnesium in their soil and drinking water. The distribution of affected and nonaffected villages indicates that communicable infectious or genetic etiology is unlikely. As a result of the isolation and primitive technology, domestic animals (except dogs and pigs) were not found among the Auyu and Jakai, and no manufactured products (including metals, ceramics, textiles, petrochemicals, medicines, food additives, condiments, paints, dyes, or solvents) were available to them.