The incidence of diabetes mellitus was noted to be 44% in a group of 55 hypertensive subjects with renal arterial stenosis. The hypertension in these individuals was quite variable, ranging from mild to malignant. Their mean age was less than 53 years. Impairment of renal function was only mild to moderate in the majority of the subjects. The high incidence can be interpreted as favoring the concept that renal-artery stenosis is often secondary to essential hypertension and an increased tendency to atherosclerosis, rather than a primary phenomenon in the development of hypertension. Alternatively, it may provide an explanation for some cases of severe hypertension among diabetics, particularly when mild hypertension becomes accelerated and severe. The theoretical and statistical problems pertaining to this interpretation are briefly discussed. The report emphasizes the need to investigate clinically for renal-artery disease in hypertensive diabetics and for diabetes in renal hypertensive subjects.