Many individuals with normal visual acuity are unable to discriminate the direction of 3-D motion in a portion of their visual field, a deficit previously referred to as a stereomotion scotoma. The origin of this visual deficit has remained unclear. We hypothesized that the impairment is due to a failure in the processing of one of the two binocular cues to motion in depth: changes in binocular disparity over time or interocular velocity differences. We isolated the contributions of these two cues and found that sensitivity to interocular velocity differences, but not changes in binocular disparity, varied systematically with observers' ability to judge motion direction. We therefore conclude that the inability to interpret motion in depth is due to a failure in the neural mechanisms that combine velocity signals from the two eyes. Given these results, we argue that the deficit should be considered a prevalent but previously unrecognized agnosia specific to the perception of visual motion.