Agarose and/or cellulose acetate electrophoresis was performed on the CSF of one thousand patients. In patients with clinically definite multiple sclerosis (MS) (N = 267) 92.8% had oligoclonal banding (O.B.). In patients with possible MS (N = 283) O.B. was present in 31.1%. In patients with other neurological diseases (N = 450) O.B. was present in 8% (N = 36). Nineteen non-MS patients with positive O.B. had serum bands or disorders known to be associated with local immune response. The remaining 17 patients had no explanation for the oligoclonal banding. In the majority of these MS had not been a diagnostic consideration. CSF electrophoresis is the single most reliable laboratory test in multiple sclerosis and deserves incorporation into the diagnostic criteria for the disease.