The majority of cases of anti-glutamic acid decarboxylase (GAD)-antibody-positive cerebellar ataxia are reported to have high levels of anti-GAD antibody, and the diagnostic value of low titers of anti-GAD antibody in a patient with cerebellar ataxia is still unknown. The purpose of this study was to verify the characteristics of low-titer-anti-GAD-antibody-positive cerebellar ataxia patients and the diagnostic value of low titers of anti-GAD antibody in patients with cerebellar ataxia. The subjects were six patients positive for low-titer GAD antibody (<100 U/mL). We examined them with MRI, including voxel-based morphometry, and with single-photon emission computed tomography and monitored the GAD antibody index in the cerebrospinal fluid. The levels of antineuronal, antigliadin, anti-SS-A, antithyroid antibodies, and of vitamins E, B1, and B12 were determined. Thoracic and abdominal CT scans were performed to exclude a paraneoplastic origin. We treated three patients with immunotherapy. All cases showed cortical cerebellar atrophy. The GAD antibody index in three of the five patients reviewed was >1.0. Two of the six patients were thyroid antibody-positive, and one was both antinuclear- and anti-SS-A antibody-positive. After the administration of immunotherapy to three patients, two showed clear effectiveness, and one, transient effectiveness. Effectiveness was greatest in the two patients with familial occurrence of the disease. In cerebellar ataxia, regardless of family history or isolated illness, it is critical to measure the GAD antibody level, and, even with a low titer level, if the result is positive, immunotherapy should be considered.