In a prospective study, 120 patients with unilateral central retinal vein occlusion (CRVO) were investigated to determine the role of the relative afferent pupillary defect (RAPD) in differentiating ischemic from nonischemic CRVO. In 87 patients with nonischemic CRVO, 90% had a RAPD 0.3 log units or less and none had a RAPD larger than 0.9 log units. In contrast, in 33 patients with ischemic CRVO 91% had a RAPD of 1.2 log units or more, and none had a RAPD smaller than 0.6 log units. Thus, this simple, quick, and inexpensive test has proved to be a highly sensitive and reliable indicator in the differential diagnosis of the two types of CRVO.