Twenty-four patients with persisting hypertension after renal artery reconstruction were re-investigated 1--8 years after surgery. They underwent renal arteriography, determination of plasma renin activity, renography and renal function studies in order to find the causes of the postoperative hypertension. Restenosis was found in 6 patients, in 3 of whom it was of functional significance according to the positive renin tests (renin ratio greater than 1.5). Positive renin tests were found in 2 other patients. One had occlusion of a renal artery branch and the other hypoplasia of the kidney due to chronic nephritis. No explanation of the persisting hypertension could be found in 19 patients at re-examination. In 10 of them, however, biopsy from the affected kidney obtained during operation showed nephrosclerosis, which may explain the outcome. Fourteen of the 19 patients had negative renin tests preoperatively. These negative tests indicate that renal artery stenosis was not the only cause of hypertension. It may be concluded that the renin test is of the utmost value in the selection of patients for renal artery reconstruction and should always be considered. A biopsy from the contralateral kidney may be necessary in order to detect other causes of hypertension than renal artery stenosis. The importance of re-investigating patients with persisting hypertension is confirmed by the present study.