Posttransplant erythrocytosis (PTE) represents a common complication in allograft recipients with normal renal function. Although the pathogenesis is not completely known, an alteration in the regulation of erythropoietin production by native kidneys or by renal allograft have been implicated as the main causes. Traditional therapies include repeated phlebotomies, bilateral native nephrectomies, and anticoagulant therapy. Recently, theophylline has been proposed as an effective therapy, although without general acceptance. Also, angiotensin-converting enzyme inhibitors have been involved in the development of anemia in chronic renal failure and dialysis patients. The aim of the present study was to demonstrate the efficacy of captopril on long-term treatment of PTE. Nineteen renal allograft recipients affected with severe PTE were included in the study. All patients had their native kidneys and none had a renal tumor or hydronephrosis. Restrictive criteria for PTE were applied to all patients and other causes of erythrocytosis were rationally excluded. Captopril was administered at a dose of 25 mg/24 hr (12.5 mg b.i.d.) during 12 months and no change on the initial dose was made during follow-up. After 3 months of captopril therapy and during the study period, significant reductions in hematocrit (P < 0.001), hemoglobin (P < 0.001), and RBC count (P < 0.001) were obtained in all patients. Erythropoietin levels decreased significantly during the study period, although the values were within the normal range of our laboratory. Captopril was well tolerated and only 1 patient had to be withdrawn from the drug because of dry cough. The present study has shown that captopril, at a low dose, represents a safe and effective therapy for PTE, without remarkable side effects or graft dysfunction. Long-term treatment with captopril in PTE did not induce anemia.