It appears well established that a microcytic, hypochromic anaemia is present in patients receiving regular haemodialysis treatment, who also suffer from chronic aluminium intoxication. This characteristic anaemia is slightly improved following deionization or reverse-osmosis treatment of dialysate water. Iron deficiency has been tentatively excluded as a cause of this anaemia by measurement of serum ferritin levels. The exact mechanisms involved in the pathogenesis of this anaemia are still to be fully elucidated but a disturbance in haem synthesis and porphyrin metabolism seems probable, and secondary effects of PTH in the bone marrow may be involved. Evidence has accumulated that aluminium is the most likely ion responsible for this anaemia but other ions, trace metals in excess or deficiency and potentially toxic substances cannot be excluded yet.