We treated 94 patients by percutaneous transluminal angioplasty (PTA) for renal artery stenosis (RAS). Prior to PTA, a renogram during angiotensin converting enzyme (ACE) inhibition with captopril was performed, but the result did not influence the decision to treat. The parenchymal time activity curves were judged by visual interpretation. Of the 94 patients, 51 had unilateral and 43 bilateral RAS. In 17 patients with bilateral RAS, PTA could be performed only in the least affected kidney; because of this the effect of PTA on their blood pressure could not be evaluated. Of the remaining 77 patients, a positive captopril renogram was seen in all 31 cured patients, in 22 of the 27 patients with improvement, and in six of the 19 patients with no change of their blood pressure. The sensitivity of the tests for cure and improvement of the blood pressure was 91% (53/58 patients) for all patients, 95% in patients with unilateral RAS (35/37), and 86% (18/21 patients) in patients with bilateral RAS, bilaterally treated. In 18 patients with a negative captopril renogram the blood pressure improved in five, and did not change in 13 patients. The success of PTA in patients with a negative captopril renogram was so poor that we feel it would have been better not to have performed angiography and PTA at all. In conclusion, captopril renography is a useful investigation in assessing the likelihood of blood pressure reduction after PTA of renal artery stenosis.