Audio frequency mastoid vibration is known to activate vestibular afferents in humans and experimental animals (1,2). However, balance responses evoked by mastoid vibration have not been reported thus far. In the present study we measured whole body responses evoked by mastoid vibration and endeavoured to determine their origin. Ten healthy subjects were studied while standing upright upon a force platform, with their feet together and eyes shut. 500 Hz bone-conducted tones of 127 dB FL (force level) and 2 s duration were delivered over the mastoid process using a B71 clinical bone vibrator. The 3-D movement of a marker attached over the C7 spinous process was measured at 200 Hz using a motion-capture system (CODA mpx 30). Ground reaction forces were measured using a force platform (Kistler). In a baseline study, right mastoid, left mastoid or null stimuli were delivered randomly to subjects facing forwards. Subjects were then studied with their heads rotated by 45 and 90 deg to the left and the right sides. Because low frequency vibration applied directly to neck muscles may produce body sway (3) we compared the response to mastoid stimulation with that evoked by stimulation directly above the external acoustic meatus or along the length of the sternocleidomastoid (SCM) muscle. Mastoid vibration evoked a force response that commenced at <200 ms and peaked at <450 ms. The medio-lateral (ML) components were oppositely directed for left and right sided stimulation while antero-posterior (AP) components had a similar direction. This resulted in oblique sway away from the stimulated ear and forwards. Peak force amplitudes (mean±SEM) of 0.86±0.19 N (ML) and 0.59±0.23 N (AP), and C7 marker displacements of 12.55±3.5 mm (ML) and 5.77±1.7 mm (AP) were significantly different from the null stimulation condition (repeated measures ANOVA; respectively, p<0.001; p=0.015; p=0.009; p=0.041). The direction of body sway measured as the angle between the point of maximum displacement and the baseline position was significantly correlated with head yaw angle (r=0.74, p<0.05). Vibration along the length of the SCM evoked lateral displacement of the body, which decreased significantly as the stimulus moved distally, away from the mastoid. Vibration over the temporal fossa directly above the external acoustic meatus evoked oblique sway similar to mastoid vibration. Unilateral mastoid vibration evokes oblique anterolateral sway similar to that observed in response to monopolar galvanic vestibular stimulation. Its dependence on head position and stimulation site suggests a vestibular origin for this response.