Post-transplant erythrocytosis afflicts 4% to 17% of renal allograft recipients and increases the incidence of thromboembolic events. Traditional therapies have included nephrectomy of native kidneys and serial phlebotomy, neither of which has proved optimal. A recent report of enalapril-associated anemia in transplant recipients led us to treat 12 polycythemic patients (packed cell volume ≥ 0.52) with low doses 2.5 to 10 mg/d) of the agent in an uncontrolled trial. Within 3 months after beginning treatment, all patients had a decline in packed cell volume from 0.57 ± 0.01 to 0.51 ± 0.01 (P < 0.001), which stabilized at 0.47 ± 0.01 after follow-up for 1 to 9 months. No patient required therapeutic phlebotomy after starting enalapril. Serum creatinine concentrations and systemic blood pressure did not change. Enalapril may be safe and effective therapy for erythrocytosis after renal transplantation.