We studied 80 patients with definite, probable, or possible multiple sclerosis, using visual and somatosensory evoked cortical potentials as indicators of manifest or silent plaques. The findings were related to the intracerebral production of immunoglobulin G expressed by the CSF IgG index. Combining the results of evoked potentials and cerebrospinal fluid (CSF) findings, the patients were reclassified. Eight of 18 patients with clinically probable multiple sclerosis were then deemed definite, and 17 of the 42 possible cases were deemed probable. No relation was found between the presumed number of lesions and the rate of intracerebral IgG synthesis.