OBJECTIVE To determine the clinical utility of using photopic negative response (PhNR) by comparing the parameters for normal, primary open angle glaucoma (POAG) and diabetic retinopathy (DR). METHODS Electroretinography (UTAS E-3000) was performed in 12 normal, 12 POAG, and 12 DR subjects. Amplitudes and implicit times for PhNR were compared among the three groups. The mean deviation (MD) and pattern standard deviation (PSD) were evaluated using standard automated perimetry (SAP). The mean retinal nerve fiber layer (RNFL) thickness and cup-disc ratio were measured using optical coherence tomography. RESULTS The a-waves and b-waves were not different among the three groups. However, compared to normal subjects, the PhNR amplitudes were reduced, and the PhNR implicit times were prolonged in the POAG and DR patients (p<0.001, p<0.001). The MD and RNFL thickness were correlated with the amplitudes and implicit times for the PhNR. CONCLUSIONS PhNR may be useful for the detection of inner retinal dysfunction, which is seen in patients who have glaucoma or diabetic retinopathy.