To investigate the possibility that aluminium may exacerbate anaemia in dialysis patients with only modest aluminium accumulation, 15 patients whose exposure to aluminium had been low were treated for three months with the aluminium chelating agent desferrioxamine, 30 mg/kg intravenously at the end of each dialysis session. Serum aluminium concentrations before treatment were 5-125 micrograms/ml. After one month of desferrioxamine, serum aluminium (including the chelate) had risen from 54.6 (SEM 11.2) to 167.0 (27.5) micrograms/l; and after three months haemoglobin had risen from 8.46 (0.70) to 10.43 (0.80) g/l. Mean cell volume and mean cell haemoglobin concentration also increased significantly. The maximum rise in haemoglobin correlated with the patients' aluminium burden as estimated by the mean serum aluminium concentration after one month of desferrioxamine therapy (r = 0.85). The greatest response to desferrioxamine occurred in patients with a baseline serum aluminium of 15-75 micrograms/l (mean increase in haemoglobin 38%). The results indicate that even the modest aluminium accumulation found in most dialysis patients has a pronounced inhibitory effect on haemoglobin synthesis. The possible toxic effect of aluminium should be considered in all anaemic dialysis patients.