We studied the photographic records of 292 eyes, including 103 eyes with branch retinal vein occlusion, 90 fellow eyes, and 99 control eyes without branch retinal vein occlusion. All arteriovenous crossings within three disk diameters of the optic disk, including the crossings at the sites of branch retinal vein occlusions, were studied. The relative positions of the crossing artery and vein could be determined at 1,939 crossings in all eyes. Crossings at which a vein crossed over an artery were a common finding (22.3% to 33.0% of crossings), but were rare at the crossings where branch retinal vein occlusions were found (2.4%). A greater proportion of arterial overcrossings was found in eyes with branch retinal vein occlusions (77.7%) compared to fellow eyes (70.6%) or control eyes (67.0%). Our data indicate that arterial overcrossings are at relatively higher risk of branch retinal vein occlusion than venous overcrossings, and that the risk of branch vein occlusion in an eye is proportional to the number of arterial overcrossings in the eye.