To evaluate the importance of the position of the artery anterior to the vein in the arteriovenous crossing to the pathogenesis of first- and second-order branch retinal vein occlusion, we studied the fluorescein angiograms of 65 patients with branch retinal vein occlusion (65 eyes). The corresponding crossing in the opposite arcade (superior or inferior) served as the control. In a statistically significant percentage of crossings, the arteries were anterior to the veins in second-order branches (odds ratio, 6.66; 95% confidence interval, 1.98 to 32.33; chi 2, 12.56; P = .000394). However, their position was not found to be important in the pathogenesis of first-order occlusions (odds ratio, 1.3; 95% confidence interval, 0.23 to 9.01; chi 2, 0.14; P = .708281). These results suggest that some differences may exist in the risk factors for branch retinal vein occlusion depending on the site of the occlusion.