Background: Little is known about adult-onset opsoclonus- myoclonus syndrome (OMS) outside of individual case reports. Objective: To describe adult-onset OMS. Design: Review of medical records (January 1, 1990, through December 31, 2011), prospective telephone surveillance, and literature review (January 1, 1967, through December 31, 2011). Setting: Department of Neurology, Mayo Clinic, Rochester, Minnesota. Patients: Twenty-one Mayo Clinic patients and 116 previously reported patients with adult-onset OMS. Main Outcome Measures: Clinical course and longitudinal outcomes. Results: The median age at onset of the 21 OMS patients at the Mayo Clinic was 47 years (range, 27-78 years); 11 were women. Symptoms reported at the first visit included dizziness, 14 patients; balance difficulties, 14; nausea and/or vomiting, 10; vision abnormalities, 6; tremor/tremulousness, 4; and altered speech, 2. Myoclonus distribution was extremities, 15 patients; craniocervical, 8; and trunk, 4. Cancer was detected in 3 patients (breast adenocarcinoma, 2; and small cell lung carcinoma, 1); a parainfectious cause was assumed in the remainder of the patients. Follow-up of 1 month or more was available for 19 patients (median, 43 months; range, 1-187 months). Treatment (median, 6 weeks) consisted of immunotherapy and symptomatic therapy in 16 patients, immunotherapy alone for 2, and clonazepam alone for 1. Of these 19 patients, OMS remitted in 13 and improved in 3; 3 patients died (neurologic decline, 1; cancer, 1; and myocardial infarction, 1). The cause of death was of paraneoplastic origin in 60 of 116 literature review patients, with the most common carcinomas being lung (33 patients) and breast (7); the most common antibody was antineuronal nuclear antibody type 2 (anti-Ri, 15). Other causes were idiopathic in origin, 38 patients; parainfectious, 15 (human immunodeficiency virus, 7); toxic/metabolic, 2; and other autoimmune, 1. Both patients with N-methyl-D-aspartate receptor antibody had encephalopathy. Improvements were attributed to immunotherapy alone in 22 of 28 treated patients. Conclusions: Adult-onset OMS is rare. Paraneoplastic and parainfectious causes (particularly human immunodeficiency virus) should be considered. Complete remission achieved with immunotherapy is the most common outcome.