Twenty-six patients with clinically nonischemic unilateral central retinal vein occlusion and four in whom clinical categorization was unclear were recruited. The scotopic electroretinogram (ERG) was recorded at presentation to a range of six different luminances. Patients were followed up for six months to assess the correlation between initial ERG response and subsequent outcome. Seven patients developed rubeosis iridis. The b-wave implicit time was the parameter most significantly associated with subsequent rubeosis, followed by b-wave/a-wave amplitude ratio and then b-wave amplitude. Neither a-wave amplitude nor implicit time was a valuable predictor. The b-wave implicit times were equally predictive of rubeosis at each of the luminances used. In individually affected eyes, a b-wave implicit time at presentation, averaged across the range of luminances used, of greater than 47.17 ms was strongly associated with later rubeosis, as was an intereye difference of greater than 7.39 ms.