In 149 eyes with central retinal vein occlusion (CRVO), we prospectively investigated the role of routine, clinical electroretinography (ERG) in differentiating ischemic (60 eyes) from nonischemic CRVO (89 eyes). Single-flash photopic and scotopic ERGs were recorded. Data for the amplitudes and implicit times of a- and b-waves and for the b-/a-wave amplitude ratio were analyzed in detail. The study revealed that the best ERG parameter (for both photopic and scotopic ERG) for differentiating ischemic from nonischemic CRVO was a subnormal b-wave amplitude (reduced to less than or equal to 60% or by greater than or equal to 1 SD from the normal mean value, or less than or equal to 64%-69% of that in the fellow normal eye), with a sensitivity of 80%-90% and a specificity of 70%-80%. ERG findings were correlated with the relative afferent pupillary defect (RAPD). An RAPD of greater than or equal to 0.7 log units showed a sensitivity of 88% and a specificity of 90% in differentiating ischemic from nonischemic CRVO. ERG and RAPD findings showed a good correlation. The combined ERG and RAPD tests could differentiate 97%-100% of ischemic from nonischemic CRVO cases, with a specificity of about 70%.