Guamanian ALS is named after the United States territory of Guam where a high percentage of ALS cases were diagnosed between the 1950s to 1960s. The number of cases found among the Chamorro people of Guam had an incidence at least 50 times higher than anywhere else. What made these cases in Guam so unique was its association with the Parkinson-Dementia Complex. People presented with symptoms similar to Parkinson's and Alzheimer's disease.
However, Guamanian ALS has also been discovered in New Guinea, Japan, and the Mariana Islands. It is believed that a diet consisting of cycad nut flour is responsible, while others have proposed a mutation in a neuronal protein is the cause.