Photopic electroretinograms (ERGs) elicited with red or white flashes of various intensities under a bright background light were recorded in 119 eyes of 77 diabetic patients and 19 normal control eyes. The implicit time and amplitude of photopic ERGs were analyzed in relation to the stage of diabetic retinopathy, ie, diabetic eyes without retinopathy, eyes with mild nonproliferative diabetic retinopathy (mild NPDR), moderate NPDR, preproliferative diabetic retinopathy (PPDR) and proliferative diabetic retinopathy (PDR). The implicit time was significantly delayed in eyes with moderate NPDR, PPDR and PDR relative to eyes with mild NPDR, eyes without retinopathy or normal eyes. The delay of implicit time significantly increased as the severity of retinopathy progressed from mild NPDR to PPDR. The amplitude was reduced in PPDR and PDR, while the correlation with the severity of retinopathy was not so significant as with the delay of implicit time. These findings suggest that the photopic ERG implicit time can be a good indicator for the objective evaluation of the severity of diabetic retinopathy ranging from mild NPDR to PPDR, where retinal laser photocoagulation should be considered.