OBJECTIVE AND BACKGROUND:: Persons with Parkinson disease (PD) show hypometric movements and make hypometric estimates of imagined actions. These deficits may be related to misestimates of the length of body parts. Our objective was to learn whether patients with PD are impaired in their estimations of their arm's length and standard units of distance. METHODS:: We tested 20 patients with PD, all on therapeutic doses of dopaminergic medications, and 13 healthy controls. In half of the trials, the participants stood so that either their right or left shoulder was adjacent to a wall; in the other half, their right or left shoulder was 5 feet from the wall. In the egocentric testing condition, they were asked to move their body toward or away from the wall to what they considered was an arm's length from the wall. In the allocentric testing condition, they were to move toward or away from the wall so that their proximal shoulder was a standard unit distance of 1, 2, or 3 feet from the wall. RESULTS:: The patients with PD made much greater hypometric (too close to the wall) errors. Since at 5 feet from the wall they had to move farther to underestimate distances, their errors cannot be explained by hypometric movements. The results did not differ significantly by egocentric or allocentric estimation, side of shoulder proximity, or side of PD onset. CONCLUSIONS:: Our findings support the idea that the egocentric and allocentric hypometria associated with PD is a perceptual rather than motor disorder. © 2013 by Lippincott Williams & Wilkins.