The reported geographical association between Alzheimer's disease and levels of aluminium (Al) in water supplies may reflect the inverse relation between Al and silicon (Si) concentrations in water, and the potential for Si to reduce the bioavailability of the metal. We tested this hypothesis using isotopic 26Al tracer administered orally to five healthy volunteers in the presence and absence of Si. Dissolved Si, at a concentration found in some water supplies (100 mumol/L), reduced the peak plasma 26Al concentration to 15% of the value obtained in the absence of Si. The results indicate that dissolved Si is an important factor in limiting the absorption of dietary Al.