Vestibulospinal reflexes are important for upright stance and locomotor control. Information from both the vestibular and the proprioceptive system must be combined centrally to guarantee appropriate compensation for a physical disturbance. Recent single-unit recordings from the monkey demonstrated vestibulo-proprioceptive interaction in the fastigial nucleus (deep cerebellar nucleus). The present study investigated whether integration of vestibular and proprioceptive signals is compromised in humans with cerebellar degeneration. Control subjects and patients were exposed to binaural, sinusoidal galvanic vestibular stimulation at 0.16 Hz, while their static head-on-trunk position was systematically altered in the head-horizontal plane from 60 degrees left to 60 degrees right. Controls responded to different head-on-trunk positions with fully compensatory changes in the direction of galvanically induced body sway, keeping it aligned with the head-frontal plane. In patients, this compensatory change was lacking. Findings support the assumption that the cerebellum plays a central role in the integration of vestibular and proprioceptive signals in humans. This form of impaired sensory interaction is probably a clinically important component of cerebellar stance and gait ataxia.