Measurement of the retinal blood pressure by means of ophthalmodynamometry is a diagnostic aid in the evaluation of the carotid circulation. The procedure usually consists of exerting pressure on the eye with the ophthalmodynamometer and noting the reading on the calibrated plunger when the central retinal artery begins to pulsate (diastole) and the reading when the artery remains collapsed (systole). When the ophthalmodynamometric values of the two eyes show a difference that exceeds 15% for diastole or 20% for systole, the carotid circulation is inferred to be compromised on the side of the lower pressure. It has been suggested, however, that inequality of intraocular pressures may invalidate such comparisons. 1-6 Nevertheless, this possibility has not heretofore been documented with a series of cases that includes specific measurements. To determine the effect that differences in intraocular pressure may have on ophthalmodynamometric measurements, we selected 12 patients with glaucoma in whom the