Children with end stage renal failure and anaemia have an increased cardiac index and often gross ventricular hypertrophy. Correction of anaemia with recombinant human erythropoietin (r-HuEpo) for less than six months results in a reduction in the cardiac index without a significant reduction in left ventricular hypertrophy. Seven children receiving dialysis (group 1) were studied to determine whether a reduction in left ventricular hypertrophy would occur after a 12 month period of r-HuEpo. A decrease in the cardiac index was seen by six months, and a significant reduction in left ventricular mass index and cardiothoracic ratio was seen by 12 months. Successful renal transplantation also results in a reduction in the cardiac index and left ventricular hypertrophy, but the relative contributions of correction of anaemia and correction of biochemical disturbance is unknown because they usually improve simultaneously. To investigate this, six children (group 2) who already had a mean haemoglobin concentration of 107 g/l while receiving dialysis were followed up for 12 months after successful transplantation. They showed no significant change in haemoglobin concentration, but a dramatic improvement in biochemistry. There was no significant change in cardiovascular function. Anaemia is the more dominant influence on cardiovascular function in end stage renal failure.