ABSTRACT: In this study, the authors investigated the link between jaw clenching/bruxism and temporal bone movement associated with multiple sclerosis (MS). Twenty-one subjects participated in this study (10 patients with MS and 11 controls). To quantify the change in intracranial dimension between the endocranial surfaces of the temporal bones during jaw clenching, an ultrasonic pulsed phase locked loop (PPLL) device was used. A sustained jaw clenching force of 100 lbs was used to measure the mean change in acoustic pathlength (∆L) as the measure of intracranial distance. In the control subjects the mean ∆L was 0.27 mm±0.24. In subjects with MS the mean ∆L was 1.71 mm±1.18 (p < 0.001). The increase in magnitude of bi-temporal bone intracranial expansion was approximately six times greater in subjects with MS compared to controls. Therefore, jaw clenching/bruxism is associated with more marked displacement of the temporal bones and expansion of the cranial cavity in patients with MS than in control subjects.