The presence of oligoclonal bands (OCB) of immunoglobulin G (IgG) is in our days the most useful finding in the study of the CSF for the diagnosis of multiple sclerosis (MS). The most sensitive method for the detection of OCB is the isoelectric focusing followed by immunoblotting. The prevalence of OCB changes in different populations with a rank of results from 60 to 95 97%. We have determined the prevalence of OCB in our population and the sensitivity and the specificity of the technique used in our laboratory. We have included 391 patients in whom we analysed the presence of OCB, subdivided in; Group 0: Diagnosed of MS, group 1: First episode of demyelinating process, group 2: Neurological disorders considered noninflammatory or nonautoimmune (NINA),group 3: Neurological disorders considered inflammatory, infectious or autoimmune (IIA). The presence of OCB was searched in CSF and serum simultaneously using isoelectric focusing and immunoblotting. In order to standardize the technique we achieved and internal and external validation. Internal validation: sensitivity and specificity (using as a control group first the group NINA and after the group IA). External validation: we choose 10 pairs of CSF/serum from patients with different diagnostics and sent to a reference laboratory ( Karolinska Institute Medical School) that was blind of our results and of the diagnostics. The prevalence of OCB in each group has been: group 0 (MS): 87.7%, group 1: 54.8%, group 2 (NINA): 17.5%, group 3(IIA): 52.7%. Sensitivity: 97.7%, specificity using group NINA as control 82.5% and using group IIA 45.7%. Concordance with the reference laboratory in 9/10 determinations. We conclude that in our population the prevalence of OCB, in patients with MS, is lower than in Northern Europe. The OCB appear in may inflammatory, autoimmune diseases, their specificity for the diagnostic of MS is low.