The serum and cerebrospinal fluid (CSF) of 8 women with ataxia, 6 of whom also had eye movement abnormalities believed to be opsoclonus, were found to contain a highly specific antineuronal antibody we call anti-Ri. Seven of the 8 women also had or developed cancer carcinoma of the breast in 5, adenocarcinoma in an axillary lymph node in 1, and carcinoma of the fallopian tube in 1. Four patients presented with the neurological disorder; the cancer was diagnosed first in the other 4. Immunohistochemical studies using serum or CSF from all 8 patients revealed a highly specific antibody interaction with central nervous system neuronal nuclei but not with glial or other cells; the titer ranged from 15,000 to 1320,000 in serum and from 12,000 to 116,000 in CSF. Biotinylated IgG from the patients' serum reacted with the tumors of 3 of 4 patients with anti-Ri antibody but not with breast cancers from patients without anti-Ri antibody. Immunoblots against cerebral cortex neuronal extracts identified protein antigens of 55-kd and 80-kd relative molecular mass. Serum titers by immunoblot ranged from 1500 to more than 140,000 and CSF titers, from 110 to 12,000. The relative amount of anti-Ri was always higher in CSF than in serum. The antibody was not present in sera from normal individuals; patients with breast cancer without opsoclonus; other patients with opsoclonus; or patients with other paraneoplastic syndromes related to breast, ovarian, or small-cell lung cancer. We conclude that the presence of anti-Ri antibody identifies a subset of patients with paraneoplastic ataxia and eye movement disorders (opsoclonus) who usually suffer from breast or other gynecological cancer; the antibody when present is a useful marker for an underlying malignancy.