This review covers the amyotrophic lateral sclerosis (ALS)/parkinsonism dementia complex (PDC) of Guam. Clinical and epidemiological characteristics, genetic possible and environmental causes, and neuropathological features of the disease are discussed.Recent studies of clinical syndromes and neuropathological studies are compared with previous descriptions of the disease. The latest genetic and environmental studies are also reviewed.In recent years, understanding of the molecular pathogenesis of neurodegenerative diseases has evolved. ALS/PDC shares neuropathological features found in many neurodegenerative diseases such as Alzheimer's disease, Lewy body disease, and frontotemporal lobar degeneration. Thus, examining ALS/PDC may provide further explanations on how various proteins seen in neurodegenerative disorders may be interrelated.