Enabling veridical spatial perception in immersive virtual environments (IVEs) is an important yet elusive goal, as even the factors implicated in the often-reported phenomenon of apparent distance compression in HMD-based IVEs have yet to be satisfactorily elucidated. In recent experiments (Interrante et al., 2006), we have found that participants appear less prone to significantly underestimate egocentric distances in HMD-based IVEs, relative to in the real world, in the special case that they unambiguously know, through first-hand observation, that the presented virtual environment is a high fidelity 3D model of their concurrently occupied real environment. We had hypothesized that this increased veridicality might be due to participants having a stronger sensation of 'presence' in the IVE under these conditions of co-location, which state of mind leads them to act on their visual input in the IVE similarly as they would in the real world (the presence hypothesis). However, alternative hypotheses are also possible. Primary among these is the visual calibration hypothesis: participants could be relying on metric information gleaned from their exposure to the real environment to calibrate their judgments of sizes and distances in the matched virtual environment. It is important to disambiguate between the presence and visual calibration hypotheses because they suggest different directions for efforts to facilitate veridical distance perception in general (non-co-located) IVEs. In this paper, we present the results of an experiment that seeks novel insight into this question. Using a mixed within- and between-subjects design, we compare participants' relative ability to accurately estimate egocentric distances in three different virtual environment models: one that is an identical match to the occupied real environment; one in which each of the walls in our virtual room model has been surreptitiously moved ~10% inward towards the center of the room; and one in which each of the walls has been surreptitiously moved ~10% outwards from the center of the room. If the visual calibration hypothesis holds, then we should expect to see a degradation in the accuracy of peoples' distance judgments in the surreptitiously modified models, manifested as an underestimation of distances when the IVE is actually larger than the real room and as an overestimation of distances when the IVE is smaller. However, what we found is that distances were significantly underestimated in the virtual environment relative to in the real world in each of the surreptitiously modified room environments, while remaining reasonably accurate (consistent with our previous findings) in the case of the faithfully size-matched room environment. In a post-test survey, participants in each of the three room size conditions reported equivalent subjective levels of presence and did not indicate any overt awareness of the room size manipulation