Ataxia-telangiectasia (A-T) is an autosomal recessive disorder characterized by progressive neurological deficits, including prominent ocular motor dysfunction. Unstable fixation often leads to difficulty reading and blurred vision. Here we characterize the disturbance of visual fixation in A-T. METHODS: We recorded eye movements from 13 A-T patients (with dual search coils in five patients and with video oculography in 7) during attempted fixation. RESULTS: Two abnormalities - nystagmus and saccadic intrusions - were common. Horizontal, vertical, and torsional nystagmus was present in straight-ahead (spontaneous nystagmus) and eccentric gaze (gaze-evoked nystagmus). In eight patients the horizontal nystagmus changed directions - periodic alternating nystagmus (PAN). Two types of saccadic intrusions occurred - micro-saccadic oscillations (SO) and square-wave saccadic intrusions (SWSI). SO were small-amplitude (0.1-0.9 degrees ) and high-frequency (14-33Hz) back-to-back horizontal saccades. The SWSI ranged between 1-18 degrees (median: 3 degrees ) with an intersaccadic interval ranging between 50-800 milliseconds (median: 300 milliseconds). The potential impact of abnormal gaze stabilization on vision was quantified. DISCUSSION: Degeneration of cerebellar Purkinje neurons disinhibit the fastigial (FOR) and vestibular nuclei (VN). Disinhibition of VN can cause nystagmus including PAN, while disinhibition of FOR can affect saccade generating mechanisms leading to SWSI and SO.