The pathological changes of Alzheimer's disease include the deposition of amyloid beta protein (A beta) as senile plaques in the brain. We hypothesized that the rapid removal of A beta s from the blood may act as a peripheral A beta drainage sink from the brain. In this study, the plasma A beta concentrations and the cognitive functions were investigated for in 57 patients on hemodailysis (69.4 +/- A 3.8 years), 26 renal-failure patients without hemodialysis (66.6 +/- 14.7 years), and 17 age-matched healthy controls (66.6 +/- 4.1 years). The concentrations of plasma A beta s increased along with the decline of renal functions. Moreover, the renal-failure patients without hemodialysis and with poorer renal functions showed lower cognitive functions. The plasma concentrations of A beta(1-42) correlated with serum creatinine (P < 0.001) and Mini-Mental-State Examination scores (P = 0.017). The dialyzers effectively removed A beta s in the blood during hemodialysis sessions. The plasma A beta concentrations showed steady or slightly decreasing along with duration of hemodialysis. The total amount of A beta s removed during a hemodialysis session was calculated to be comparable to the A beta s dissolved in the blood and the cerebrospinal fluid. The MMSE scores of the hemodialysis patients showed no clear decrease in longer hemodialysis duration. Therefore, the therapeutic approach for Alzheimer's disease by removing A beta s from the blood is worthy of further investigation, including whether or not A beta s in the brain decrease.