As 3D technology spreads, 3D imagery is being viewed in more diverse situations. Already, televisions and mobile devices are able to present 3D images, in addition to the cinema. Of these, mobile devices have the unique characteristic of requiring users to view images at close range. The authors conduct an experiment to examine the relationship between visual comfort and an individual observer's near phoria and interpupillary distance. The results show that observers with a higher degree of exophoria tend to prefer stereo images behind the screen because they cause less visual fatigue. On the other hand, observers with little exophoria or esophoria tend to prefer stereo images in front of the screen. Further, the results also show that observers with a greater interpupillary distance tend to prefer stereo images behind the screen. These findings suggest that the ability to adjust the depth of stereo images in advance based on personal phoria or interpupillary distance might help users achieve comfortable stereo viewing on mobile devices.