Six patients with acute renal failure, in five cases due to acute crescentic glomerulonephritis and in one case due to total bilateral renal cortical necrosis, were studied. All had serum erythropoietin (EPO) concentrations in the normal range, despite a relatively severe anaemia. Half-life and plasma clearance of intravenously injected recombinant human erythropoietin (rhEPO) were determined. The results indicate that the lack of compensatory increase in serum EPO to the anaemic stimulus is not due to increased catabolism, but to decreased synthesis of the renal hormone. Two patients were treated with rhEPO (Eprex). In marked contrast to untreated controls, both patients responded with vigorous reticulocytosis and normalization of haemoglobin levels while they were still in severe renal failure. These results are similar to our previous findings in patients with acute renal failure due to tubular necrosis. Under all three conditions the defective EPO synthesis is probably the dominant pathogenetic factor for the largely aregeneratory anaemia of prolonged cases, and for the sluggish restoration of red cell mass during recovery of renal function. It is concluded that defective synthesis of EPO is not only a permanent and irreversible feature of severe chronic renal failure, but that it is also present, usually in a transient and reversible form, in different types of acute renal failure.