To find out whether diseased native kidneys can be the cause of hypertension in patients whose allograft otherwise seems to function well, six such hypertensive renal transplant patients were investigated before and 4.5 +/- 1.5 (+/- SD) months after removal of their native kidneys. After nephrectomy mean arterial pressure fell considerably yet renal (allograft) plasma flow increased by 77% and vascular resistance of the allograft fell by 55%. Mean transverse cardiac diameter and electrocardiogram voltage measurements of left ventricular hypertrophy both improved significantly. Since the rise in renal plasma flow could be induced by giving captopril, an inhibitor of angiotensin II formation, to patients who had not had their native kidneys removed, the native kidneys seem to exert their effect on the allograft via the renin-angiotensin system. The improvement in allograft plasma flow after nephrectomy was maintained for more than 1 1/2 years. Administration of captopril after native nephrectomy did not further change allograft plasma flow. The findings suggest that in many carefully selected patients with post-transplantation hypertension native kidney nephrectomy offers tangible benefits.