The long-term impact of erythropoietin (EPO) treatment on cardiac structures and function was prospectively studied in eight hypertensive (Group I) and seven normotensive (Group II) patients on hemodialysis (HD). Doppler-echocardiograms were done before EPO and at two and twelve months of treatment. Mean hemoglobin (+/- SD) before EPO was 6.4 +/- 0.9; it rose significantly up to two months and then remained constant. At two months, cardiac index (CI) had significantly decreased, while peripheral vascular resistances increased. Five patients required increased antihypertensive drug treatment. No changes were seen in myocardial parameters at this short follow-up. After one year, left ventricular mass index (LVMi) decreased (p less than 0.05) in both groups concomitantly with a decrease in diastolic diameter and septum and posterior wall thicknesses. Basal LVMi was higher in Group I than in Group II, and after one year the regression was more marked in Group II. Left cardiac work showed prompt and steady improvement in both groups. Maintained partial correction of anemia with EPO during one year was associated with a return to normal of high CI, decreased left cardiac work and impressive regression of left ventricular hypertrophy.