Strong misalignment effects are found in three-dimensional (3-D)versions of Poggendorff displays viewed binocularly. The components of the standard 2-D Poggendorff figure-the parallels and the oblique segments-were presented in 3-Ddepth as a flat rectangular object with occluding edges and an oblique line situated behind the object. Three experiments investigated the misalignment effects under three different observation instructions: Subjects were told to look at the oblique (Experiment 1), at the rectangle (Experiment 2), or at the background (Experiment 3). Experiments 1 and 2 examined the effects on judgments of alignment of varying the distance in depth that separates the oblique from the rectangle. Experiment 3 examined the effects of varying the distance between the fixated background and the 3-DPoggendorff figure. Both standard and reversed misalignment effects were obtained. When the viewing condition produces crossed disparity for the oblique, perceived misalignment occurs in the usual Poggendorff direction, but it is reversed with uncrossed disparity. Moreover, the amount of misalignment is related to the amount of disparity, and it can be much stronger than is usual in the 2-D versions of the Poggendorff. The misalignment effects can be explained by binocular integration to produce a single cyclopean image.In the Poggendorff illusion (Figure 1), the oblique lines erroneously appear to be misaligned. What we show here is that misalignment effects also appear in solid (three-dimensional, or 3-D) binocular versions of Figure 1. We show how the effects depend on factors arising during binocular integration. There is a variety of contradictory reports on binocular versions of the Poggendorff in the literature, and we show how each of the varied reports can be obtained and explained by standard principles of stereopsis.While sometimes it has been considered to be due to depth perception, Poggendorff misalignment has only rarely been associated with binocular vision. Fisher and Lucas (1969), Schiffman (1990) report that pictures of real-life settings, such as an oblique tree behind an upright tree, create the same effect as the Poggendorff figure. However, the observations were undertaken only with flat pictures, and possible binocular versions, though noted by these authors, were not assessed. Pieron (1955) tried explicitly to observe the Poggendorff effect in a fully 3-D world. He observed that the effect failed there! He stated that if the two segments ofthe oblique line are made ofrope passing behind a column and serving to pull a bucket out of a well, the illusion disappears completely. Julesz (1971) examined several illusions in random-dot stereograms. In particular, he observed that when the Poggendorff rectangle is sepWe would like to thank P. Bennett for his comments on an early version of this paper. Correspondence should be addressed to C.