Recombinant human erythropoietin is a major advance in the management of patients with chronic renal failure. The sustained dose-dependent rise in haematocrit which it produces effectively abolishes symptoms of anaemia, but at the cost of an increase in blood viscosity. This in turn predisposes to increased vascular resistance and the development of hypertension. Over half of all deaths of patients with end-stage renal failure are from cardiovascular disease, notably myocardial infarction, heart failure, and stroke, for which hypertension is a known risk factor. Erythropoietin-related increases in blood pressure are therefore of particular concern, and seem to be most severe in previously hypertensive patients. There is now a need to establish the optimum rate and extent of rise of haematocrit required to alleviate symptoms without incurring undue risk.