In a low water aluminium area ( less than or equal to 0.4 micromoles/1) plasma aluminium levels were normal in 31 non-dialyzed and 7 peritoneally-dialyzed patients unless oral aluminium hydroxide was prescribed. Plasma aluminium levels were above normal in 26 patients on maintenance hemodialysis, but when aluminium hydroxide intake stopped there was a decrease in pre-dialysis plasma aluminium. Poor compliers (indicated by plasma phosphate levels) showed no change. Oral aluminium hydroxide can make a major contribution to plasma aluminium levels in patients with renal failure and should be treated with the same caution as high dialyzate aluminium. Post-dialysis plasma aluminium levels were always higher than pre-dialysis levels both on and off aluminium therapy despite the low dialyzate aluminium concentration. This rise may be due to tissue redistribution.