Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system. We compared the diagnostic performance of isoelectric focusing (IEF) combined with IgG immunoblotting to high-resolution agarose gel electrophoresis for the detection of intrathecal synthesis of IgG due to the MS disease process. In 20 patients with definite MS, IEF and high-resolution agarose gel electrophoresis had sensitivities of 90% and 60%, respectively. In the 51 patients with no evidence of MS, the methods had specificities of 94% and 96%, respectively. With a prevalence of 15% in this test population, IEF and high-resolution agarose gel electrophoresis had positive predictive values of 73% and 73% and negative predictive values of 98% and 93%, respectively. The IEF method improves the sensitivity and the negative predictive value of the oligoclonal-banding assay, the IEF gels are easier to interpret, and the IEF assay requires a smaller cerebrospinal fluid volume.