When you cannot measure, your knowledge is meager and not satisfactory. —Lord Kelvin. There are a number of changes visible in the retinal arteries and veins of patients with early or late hypertension. The most frequent are narrowing of the arteries (spasm), changes in caliber (so-called spasm or atheroma), tortuosity, mottling of the arterial reflex, "silver wire" arteries and arteriovenous nicking. We have chosen the last of these lesions for study because (1) arteriovenous nicking is easily and accurately identified after one has sufficient practice, (2) it is the most constant change present in all forms of long-standing hypertension and (3) it never occurs except in patients who have or have had hypertension. Wagener has stated that "spasm" never occurs except in hypertensive patients. Our experience has confirmed this repeatedly, but the phenomenon is largely confined to patients with exceedingly high diastolic pressure (so-called malignant hypertension) and consequently is not