PURPOSE: To report the ophthalmic findings and response to treatment in a patient with glutamic-acid decarboxylase antibodies. DESIGN: Case report. METHODS: A 55-year-old woman developed progressive, painful, low back muscle spasms, vertical diplopia, downbeating nystagmus, and asymmetric appendicular ataxia. RESULTS: Downbeating nystagmus was present in primary gaze with an alternating skew deviation in lateral gaze. Serum and cerebrospinal fluid GAD antibodies were detected. Treatment with diazepam led to resolution of spasticity, whereas repeated courses of intravenous immunoglobulin improved cerebellar function, including appendicular ataxia and downbeating nystagmus. CONCLUSIONS: Patients with GAD antibodies may have elements of both Stiff-person syndrome (muscle rigidity and spasms) and prominent cerebellar dysfunction. Treatment with diazepam rapidly improved Stiff-person symptoms, whereas IVIg was partially effective at the early stage of cerebellar dysfunction.