The results of cerebrospinal fluid agarose gel electrophoresis in 300 consecutive patients were correlated with neurological examinations and diagnoses, other cerebrospinal fluid studies, and the results of evoked potential examinations. The presence of oligoclonal bands was the most sensitive test for multiple sclerosis (MS); bands were present in from 100% (11/11) of patients with definite MS to 82% (27/33) of those with possible MS (classified by McAlpine criteria). The visual evoked response was the next most sensitive study. Thirty-eight patients without MS or related disorders had bands in the IgG region. Three patients had plasma cell dyscrasias. Seven patients had thick single bands. Single bands did not correlate with chronic polyneuropathy but appeared to be an artifact of storage. Twenty-eight patients had active neurological disease, including cerebral infarction (in 5), viral infection (in 4), remote effect of carcinoma (in 4), and acute and chronic polyneuropathies (in 6). In acute illnesses (i.e., vascular insults), repeat electrophoresis showed disappearance of bands. In continually active disease, bands persisted. These results indicate that the presence of oligoclonal bands provides sensitive supporting evidence for the diagnosis of MS but that bands may be present in other disorders, including those not directly related to infection or abnormal immune response. The data suggest that oligoclonal bands may represent an immune response to neurological injury that is prominent in disorders with a particularly intense or continuous antigenic stimulus.