BACKGROUND The unregulated renin release by native kidneys is one of the factors responsible for the high incidence of hypertension after renal transplantation but, even after three decades of transplantation, there is still a lack of a method to identify it as the major cause of hypertension. METHODS We investigated whether or not peripheral renin activity, before and 90 min after 25 mg of captopril, can play this role. One hundred and five consecutive patients with SCr less than 2 mg/dl were studied 18 +/- 8 months after renal transplantation. Forty-seven of them were considered hypertensive and 58 normotensive. All hypertensive patients were submitted to the captopril test to analyse the peripheral renin activity profile. RESULTS In the hypertensive group, 17 patients (36%) were considered Renin-pos and 30 (64%) Renin-neg. All Renin-pos (stimulated renin = 19.1 +/- 6.4 ng/ml/h) patients were submitted to bilateral nephrectomy (bNx) and re-evaluated 6 months later. All of them normalized renin activity (4.4 +/- 3.0 ng/ml/h, P = 0.0001) and 10 of 17 (60%) became normotensive and off drugs. The remaining seven (40%) decreased the number of hypotensive drugs from 2.2 +/- 0.5 to 0.5 +/- 0.7/pt/day. There was a correlation between b-Renin and dBP (r = 0.47, P < 0.05) which was lost after bNx. An 'in situ' renal-cell carcinoma was found in two cases. Serum creatinine did not change. CONCLUSIONS This study shows that the unregulated renin-angiotensin system from the native kidneys plays a major role in the maintenance of hypertension in some patients with normal graft function and that peripheral renin activity can identify those who will benefit from bilateral nephrectomy.