Erythropoietin (EPO) is the main regulatory hormone for the control of erythropoiesis. EPO leads to enhanced mitosis and differentiation of erythroid precursors in the bone marrow. The major stimulus for EPO-formation is anaemia of various origin, resulting in an exponential relation between EPO levels and a decrease in haematocrit. Another important stimulus for increased EPO production is a fall of the arterial oxygen tension caused by either cardiopulmonary disorders or by a decrease of the oxygen tension in the inspiratory gas. Human erythropoietin was first isolated and purified from a large amount of urine of patients with aplastic anaemia. After the EPO gene had been cloned and expressed, biotechnically produced recombinant human erythropoietin (rHu-EPO) became available for clinical trials. EPO deficiency appears to be the major cause of renal anaemia, and hence the treatment of these patients is the most important indication for clinical use. Encouraging results in patients whose anaemia is not of renal origin have also been reported, using treatment with rHu-EPO. In preoperative autologous blood donation programmes prior to elective surgery, rHu-EPO therapy improved the amount of donated blood and ameliorated the decrease of haematocrit values. Side effects such as hypertension, thrombosis, hypercalcaemia, elevated liver enzymes were rare and were mostly related to the underlying disease.