The case is reported of a young woman with severe hypertension, unilateral renal artery stenosis, variously normal or marginally high plasma concentrations of active renin, angiotensin II, aldosterone, sodium, and potassium; and normal total exchangeable and total body sodium and potassium. Arteriograms and ureter catheterization showed the stenosis to be severe, but the unstimulated renal vein renin and angiotensin II differential to be modest. Captopril caused an initial fall in angiotensin II and arterial pressure. During prolonged captopril treatment, plasma angiotensin II and aldosterone remained depressed; exchangeable and total body sodium and potassium were unaltered. Blood pressure fell further to normal levels during prolonged captopril treatment, while subsequent surgical correction of the renal artery stenosis was curative; absolute values of blood pressure and plasma angiotensin II were similar in both situations. The findings support, without proving, the concept that chronic modest elevation of angiotensin II may be responsible for sustained hypertension in unilateral renal artery stenosis. Patients of this type contrast sharply with those, also with severe renal artery stenosis or occlusion, who have gross elevation of renin, angiotensin II, and aldosterone, with sodium and potassium deficiency. Captopril or surgery are effective in both syndromes, but the manner of response to treatment differs markedly.