This paper describes clinical features of high renin hypertension in the elderly. Peripheral plasma renin activity ranged from 0 to 20.1 ng/ml/hr in 59 hypertensive in-patients aged 70 to 86. The patients were divided into 2 groups: 9 cases with plasma renin activity greater than or equal to 3.0 ng/ml/hr (high renin group) and the remaining 50 with plasma renin activity less than 3.0 ng/ml/hr (control group). The development of hypertension differed between the 2 groups. Six of the high renin group (66.7%) had a history of acceleration of previously mild hypertension, while only 3 of the control group (6.0%) had this history (p less than 0.01). The frequencies of high diastolic blood pressure (greater than or equal to 120 mmHg), massive proteinuria (at least 3.0 g/day), hypokalemia (serum potassium less than or equal to 3.0 mEq/L) and high serum cholesterol (greater than or equal to 250 mg/100 ml) were significantly greater in the high renin group than in the control group (p less than 0.01, respectively). Renovascular hypertension was suspected in 6 patients from the high renin group (66.7%), as compared with 1 of the control group (2.0%) (p less than 0.001). There was massive proteinuria in 3 of 6 patients with renovascular hypertension in the high renin group and 2 showed nephrotic syndrome. Thus, two-thirds of the elderly patients with high renin hypertension had probable renovascular hypertension with a history of rapid progression of hypertension.