Oligoclonal immunoglobulin D (IgD) bands were observed in 20 of 110 CSF samples investigated by our method for isoelectric focusing (IEF) of IgD in unconcentrated CSF. These were mostly from demyelinating disorders, viral CNS infections, and CNS tumors. The patterns of expression of IgD were similar to that of IgG, except in cases with tumors where oligoclonal IgD may occur independently of IgG. The study shows for the first time the presence of oligoclonal IgD in CSF from neurologic patients.