Evidence for the involvement of endothelial cells in the pathogenesis or erythropoietin-induced hypertension, and for endothelial cell damage in patients with chronic renal failure, has emerged and appears to be of major concern. We, therefore, investigated the effect of recombinant human erythropoietin (rHuEPO) therapy on endothelium-derived hormones in predialysis patients with progressive renal anemia. At the entry to the trial, the serum thrombomodulin concentration (Tm) and plasma endothelin-1 concentration (ET-1) in the predialysis patients were significantly higher than those in age- and sex-matched normal subjects. Following a 16 week period of treatment with 6000IU rHuEPO given intravenously once a week, patients' hematocrit increased from 27.1 +/- 2.6% to 34.6 +/- 3.2% (n = 16, P < .001). A positive correlation was found between Tm and serum creatinine concentration (Cr) (r = 0.61, P < .05 (n = 16), but no correlation was found between ET-1 and Cr. Tm and Tm/Cr significantly decreased from 7.9 +/- 2.8 ng/mL to 6.6 +/- 2.4 ng/mL (P < .01, n = 16), and from 2.1 +/- 0.7 (x10(-10) to 1.6 +/- 0.7 (x10(-10), P < .01, n = 16), respectively. However, there was no change in ET-1 as a result of the rHuEPO therapy. Creatinine clearance (Ccr), Cr, total amount of daily Tm excretion, Tm clearance/Ccr, daily urinary protein and albumin excretion, and blood pressure also remained unchanged throughout the trail. The present study indicates that correcting anemia by rHuEPO therapy reduces an abnormally elevated Tm in predialysis patients while blood pressure and renal function remain unchanged, suggesting that rHuEPO has a beneficial effect on endothelial cell dysfunction in chronic renal failure patients. This effect may be mediated via an improved oxygen supply to the endothelial cells due to the amelioration of anemia by rHuEPO.