The unusually high incidence of amyotrophic lateral sclerosis/Parkinson-dementia complex (ALS/PDC) among the Chamorro people of Guam has fueled an intense search for the etiologic agent responsible for this neurodegenerative disease. Recently, a biomagnification hypothesis was proposed to account for the role of dietary consumption of beta-methylamino-alanine (BMAA) in patients with ALS/PDC. However, this hypothesis is hotly debated and a direct association between BMAA and neuronal injury in vivo has been lacking. We provide evidence that introduction of BMAA into the CNS of mice leads to sporadic death of hippocampal neurons, supporting a direct causal link between BMAA and neuronal injury.