Purpose. To examine retinal function in chronic ocular ischemia using multifocal electroretinography (mfERG). Methods. Thirteen patients with unilateral ocular ischemic syndrome (OIS) underwent assessment of ophthalmic systolic blood pressure by ocular pneumoplethysmography, carotid artery patency by ultrasonography, intraocular pressure (IOP) by applanation tonometry, retinal perfusion by fluorescein angiography, and retinal function by mfERG. Results. Ophthalmic systolic blood pressure was 67.0 +/- 11.6 mm Hg in eyes with OIS and 106.1 +/- 18.0 mm Hg in fellow eyes, whereas IOP was 13.8 +/- 3.2 and 14.4 +/- 1.7 mm Hg, respectively. Summed mfERG implicit times (N1, P1, N2) were prolonged in eyes with OIS, by 7.6%, 6.2%, and 7.5%, respectively, compared with fellow eyes (P < or = 0.0048). The retardation of retinal function was significant outside the macula, whereas the assessment of responses from the central retina was limited by high variance. Second-order kernel (first slice) summed implicit times (N1, P1, N2) were also prolonged in OIS, by 6.6%, 7.3%, and 6.8%, respectively (P < or = 0.0058). Of the amplitudes, only the second-order N2 amplitude was significantly abnormal, being reduced by 23.2% in OIS (P = 0.011). Conclusions. The function of the outer and middle layers of the retina was found to be suppressed in chronic ocular hypoperfusion. The moderate delay in retinal function does not appear to explain the prominent photopic symptom of diffuse glare in bright light, and the delay could be evidence of a functional adaptation that serves to maintain and optimize signaling under conditions of compromised perfusion. (ClinicalTrials.gov number, NCT00403195.).