Cerebellar ataxia is an uncommon manifestation of hypothyroidism with unknown pathomechanism. The few descriptions of the clinical phenotype range from limb, gait, and trunk ataxia to various ocular motor abnormalities. We evaluated a 62-year-old woman with previously undetected severe hypothyroidism who presented with prominent saccadic intrusions and gait ataxia. She had high titers of antithyroid autoantibodies and anti–glutamic acid decarboxylase (anti-GAD) antibodies. Horizontal eye movement recordings revealed a series of nearly continuous pseudoharmonic square wave jerks (SWJs) constituting a square wave oscillation. Amplitudes reached maximum values of about 4, and wave frequency approached 100 cycles per minute. Thyroxine substitution and corticosteroid administration had little effect on SWJ parameters. The square wave oscillation nearly completely resolved after a single treatment session with intravenous immunoglobulin suggesting a causal link between an autoimmune process and the cerebellar dysfunction. Current concepts of the genesis of saccadic intrusions favor a role for anti-GAD antibodies in the etiology of SWJs.