Despite the growing popularity of frameless image-guided surgery systems, stereotactic head frame systems are widely accepted by neurosurgeons and are still commonly used to perform stereotactic biopsy, functional procedures, and stereotactic radiosurgery. In this study, we investigate the accuracy of the Cosman-Roberts-Wells (CRW) stereotactic frame system when the mechanical load on the frame changes between pre-operative imaging and the intervention due to different patient position - supine during imaging, prone during intervention. We analyze CT images acquired from 12 patients who underwent stereotactic biopsy or stereotactic radiosurgery. Two CT images were acquired for each patient, one with the patient in the supine position and one in the prone position. The prone images were registered to the respective supine images using an intensity-based registration algorithm, once using only the frame and once using only the head. The difference between the transformations produced by these two registrations describes the movement of the patient's head with respect to the frame due to mechanical distortion of the latter. The maximum frame-based registration error between supine and prone positions was 2.8 mm, greater than 2 mm in two patients, and greater than 1.5 mm in five patients. Anterior-posterior translation is the dominant component of the difference transformation for most of these patients. In general, the magnitude of the movement increased with brain volume, which is an index of head weight. We conclude that in order to minimize frame-based registration error due to a change in the mechanical load on the frame, frame-based stereotactic procedures should be performed with the patient in the identical position during imaging and intervention.