Serum aluminium was monitored in 19 patients admitted with acute oligo-anuric renal failure. The maximum serum aluminium obtained during the course of treatment was greater (p < 0.05) in 4 patients treated by haemodialysis alone, mean ± sem 3.78 ± 0.71 μmol/l than in 4 patients treated only by haemofiltration, 0.60 ± 0.22 μmol/l. For those 11 patients treated by both haemodialysis and haemofiltration, the maximum serum aluminium was greater (p < 0.05) during treatment with haemodialysis, 2.7 ± 0.62 μmol/l than during treatment with haemofiltration, 1.36 ± 0.15 μmol/l. There was a significant positive correlation between the maximum serum aluminium during treatment with haemodialysis and the number of hours of haemodialysis given (r = 0.76, p < 0.001). There was no significant increase in serum aluminium due to the administration of human albumin solutions. The aluminium content of dialysate water represents a major source of aluminium in patients with acute renal failure; prevention by reverse-osmosis water purification is recommended.