Few environmental risk factors for Alzheimer's disease have been identified. This lack of information may reflect the fact that salient factors affect most of the population in developed countries. Furthermore, the critical period of exposure may be earlier than hitherto suspected, during the first years of life, as the brain differentiates and develops. Exposure to lead at levels lower than those associated with evident toxicity causes mild intellectual impairment in childhood. I hypothesize that this may be one of the childhood exposures that also confers an additional risk for the onset of Alzheimer's disease.