Baseline and single-dose captopril scintigraphy with 1 mCi of 99mTc-diethylenetriamine-pentaacetic acid (or 99mTc-glucoheptonate) was performed in 5 neonates with renovascular hypertension. Unilateral renal artery thrombosis and/or renal infarction was associated with severe impairment or lack of function on both studies (3 patients). Renal ischemia due to aortic thrombus manifested itself as lack of function only following captopril (2 patients). This approach predicted renal failure as a side effect of captopril therapy in 2 patients, 1 with unilateral (contralateral kidney infarcted) and the other with bilateral renal ischemia from aortic thrombus. Single-dose captopril scintigraphy may be a useful tool to predict tolerance to captopril therapy.