Experimental animal models suggest that modulation of the amplitude and direction of vestibular reflexes are important functions of the vestibulo-cerebellum and contribute to the control of gaze and balance. These critical vestibular functions have been infrequently quantified in human cerebellar disease. In 13 subjects with ataxia telangiectasia (A-T), a disease associated with profound cerebellar cortical degeneration, we found abnormalities of several key vestibular reflexes. The vestibulo-ocular reflex (VOR) was measured by eye-movement responses to changes in head rotation. The vestibulo-collic reflex (VCR) was assessed with cervical vestibular-evoked myogenic potentials (cVEMPs), in which auditory clicks lead to EMG activity of the of the sternocleidomastoid muscle. The VOR gain (eye velocity/head velocity) was increased in all subjects with A-T. An increase of the VCR, paralleling that of the VOR, was indirectly suggested by an increase in cVEMP amplitude. In A-T subjects, alignment of the axis of eye rotation was not with that of head rotation. Subjects with A-T thus manifested VOR cross-coupling; abnormal eye movements directed along axes orthogonal to that of head rotation. Degeneration of the Purkinje neurons in the vestibulo-cerebellum probably underlies these deficits. This study offers insights into how the vestibulo-cerebellum functions in healthy humans. It may also be of value to the design of treatment trials as a surrogate biomarker of cerebellar function that does not require controlling for motivation or occult changes in motor strategy on the part of experimental subjects.