The practical value of renin secretion studies in hypertension associated with unilateral kidney disease, other than renal artery stenosis, has not been documented. This study, comprising 19 patients of this kind, disclosed three who had an abnormal renin secretion from the diseased kidney. The level of peripheral renin under basal conditions, and the change from this level as a result of provocation of renin secretion, were used to evaluate the importance of an arteriovenous renin gradient in the diseased kidney. The three patients were the only ones to become normotensive when the diseased kidney was removed in seven of the cases studied. When nephrectomy is considered in severe hypertension with unilateral kidney disease, there is a place for renin secretion studies, but a screening procedure is advisable. Measuring peripheral renin under basal conditions and after provocation of renin secretion, should reveal whether the renin-angiotensin system might be playing a part in maintaining the high BP. The finding of diminishing kidney function in many of the patients, despite good BP control, emphasizes the importance of sparing kidney function whenever possible.