Oligoclonal immunoglobulin (Ig) bands were found in cerebrospinal fluid (CSF) by agarose gel electrophoresis and thin-layer polyacrylamide gel isoelectric focusing in 10 patients with cerebral infarction and 2 patients with transient ischemic attacks. Immunofixation revealed that the oligoclonal Ig was of the G class. One patient also had a band of free lambda light chains. The appearance of oligoclonal IgG during the course disease was observed in one patient, and the disappearance in six patients. Only three patients had elevated CSF IgG levels or abnormal synthesis rate of IgG in the nervous system. The oligoclonal reaction observed in acute cerebrovascular disease may reflect a polyclonal B-cell activation within the central nervous system after brain tissue damage.