Agarose isoelectric focusing was used to demonstrate oligoclonal bands in cerebrospinal fluid (CSF) and serum from 998 consecutive neurological patients. Compared with agarose electrophoresis, agarose isoelectric focusing was slightly more sensitive, showing more (and more easily discernible) oligoclonal bands. Agarose isoelectric focusing, which has good reproducibility, revealed oligoclonal bands in CSF in 95% of 43 patients with multiple sclerosis, 44% of 39 with aseptic meningoencephalitis, and 14% of 906 with other neurological diseases. Interestingly, oligoclonal bands were found in CSF from 12% of 162 patients with acute cerebral infarction and 23% of 53 with polyneuropathy, and also in 29% of 17 with dementia, while only 4% of 206 patients with headache, vertigo, or psychoneurosis had this CSF abnormality. We recommend this procedure for the routine examination of paired CSF and serum specimens for the presence of oligoclonal bands.