Idiopathic sporadic ataxia (ISA) is the clinical term for nonfamilial ataxia with adult-onset and a slowly progressive course. However, immune-mediated cerebellar ataxia cannot be completely excluded from ISA. The current study investigated the neuropil antibodies against cell-surface antigens and clarified the clinical features and neuroimaging findings of patients with these antibodies. Using tissue-based immunofluorescence assays (TBAs), we examined antibodies against the cerebellum in serum samples from 67 patients who met the ISA diagnostic criteria, including 30 patients with multiple system atrophy with predominant cerebellar features (MSA-C) and 20 patients with hereditary ataxia (HA), and 18 healthy control subjects. According to the TBA results, we divided subjects into three groups: subjects positive for neuropil antibodies, subjects positive for intracellular antibodies only, and subjects negative for antibodies. We compared clinical features and neuroimaging findings in ISA patients among these three groups. The prevalence of neuropil antibodies in ISA (17.9%) was significantly higher than that in MSA-C (3.3%), HA (0%), or healthy subjects (0%). The neuropil antibody-positive ISA patients showed pure cerebellar ataxia more frequently than the other ISA patients. Two neuropil antibody-positive patients showed significant improvement of cerebellar ataxia after immunotherapy. We detected neuropil antibodies in 17.9% of ISA patients. Characteristic clinical features of neuropil antibody-positive ISA patients were pure cerebellar ataxia. Some cases of neuropil antibody-positive ISA responded to immunotherapy.