Abstract Selective intrarenal catheterization was used to obtain venous blood draining a lesion affecting only a segment of one kidney in nine hypertensive patients (four segmental infarcts, four renal-artery branch stenoses and one renin-secreting tumor). The main renal-vein renin ratio between the affected and contralateral kidneys (Ra/Rc) was > 1.5 in only four of nine patients (mean ratio ± S.E. 2.1 ± 0.5), whereas the ratio when the segmental sample (Rseg/Rc) was used was significantly higher: > 1.5 in all (4.5 ± 1.1). By contrast, neither plasma renin activity in segmental samples from normal areas of the affected kidney nor that in intrarenal samples of two patients with main renal-artery stenosis exceeded plasma renin activity of the affected kidney. Selective renal-vein renin sampling can specifically identify a localized source of renin that may be overlooked by main renal-vein sampling and facilitate the detection of patients with surgically remediable lesions. (N Engl J Med 290:1153–1157, 1974)