To investigate the effect of aluminum and silica in drinking water on the risk of dementia and Alzheimer's disease, the authors analyzed data from a large prospective cohort (Paquid), including 3,777 subjects aged 65 years and over living at home in 75 civil parishes in Gironde and Dordogne in southwestern France in 1988-1989. The subjects were followed for up for 8 years with an active search for incident cases of dementia or Alzheimer's disease. Mean exposure to aluminum and silica in drinking water was estimated in each area. The sample studied included 2,698 nondemented subjects at baseline, for whom components of drinking water and covariates were available. A total of 253 incident cases of dementia (with 17 exposed to high levels of aluminum), including 182 Alzheimer's disease (with 13 exposed to high aluminum levels), were identified. The relative risk of dementia adjusted for age, gender, educational level, place of residence, and wine consumption was 1.99 (95 percent CI: 1.20, 3.28) for subjects exposed to an aluminum concentration greater than 0.1 mg/liter. This result was confirmed for Alzheimer's disease (adjusted relative risk = 2.14, 95 percent CI: 1.21, 3.80). However, no dose-response relation was found. Inversely, the adjusted relative risk of dementia for subjects exposed to silica (> or = 11.25 mg/liter) was 0.74 (95 percent CI: 0.58, 0.96). These findings support the hypothesis that a high concentration of aluminum in drinking water may be a risk factor for Alzheimer's disease.