Fifty-two patients with postrenal transplant erythrocytosis were treated with an angiotensin-converting enzyme inhibitor (lisinopril or enalapril) for a median of 13 months (range 0-44). A significant fall in haemoglobin of 1.8 +/- 1.6 g dl-1 (range - 0.8 to 6.6) occurred over the first 3 months (p < 0.0001). The haemoglobin then remained stable for as long as 3 years. Both enalapril and lisinopril were equally effective. Therapy was withdrawn in 16 patients (31%) because of decline in renal function (6), anaemia (5), hypotension (3), hyperkalaemia (1) or erectile impotence (1) - complications which were all reversible. Angiotensin-converting enzyme inhibitors in low dose are a safe and effective long-term therapy for postrenal transplant erythrocytosis.